Gasoline and Diesel Fuel Injection: Operation

Gasoline and Diesel Fuel Injection: Operation
Gasoline and Diesel Fuel
Injection: Operation,
Diagnosis, and Service
OBJECTIVES: After studying Chapter 29, you should
be able to:
1. Prepare for the interprovincial Red Seal certification
examination in Appendix VIII (Engine Performance)
on the topics covered in this chapter.
2. Describe how to check an electric fuel pump for
proper pressure and delivery volume.
3. Explain how to check a fuel-pressure regulator.
4. Describe how to test fuel injectors.
5. Explain how to diagnose gasoline fuel-injection
6. Explain the differences between throttle-body, port
and direct fuel-injection.
7. Describe the operation of a diesel mechanical fuelinjection system.
8. Describe the operation of an electronic diesel fuelinjection system.
Electronic gasoline fuel-injection systems from the
mid 1970s to the mid 1990s and later are all very similar in design and operation because they share a common beginning with Robert Bosch GmbH, a German
company. Bosch is the largest supplier of fuel-injection
systems in the world. It doesn’t matter if it’s a Fiat,
Ferrari or a Ford, a Nissan, Toyota, Chevrolet, or a
Chrysler, Bosch either designed or supplied the complete system or some components in the system. In the
mid-1990s, automobile manufacturers began moving
away from the original Bosch design and we now see
many variations. However, Bosch is still a major component supplier for many of these late systems.
Most fuel-injection systems are closed loop, which
means that any fuel sent to the injectors and not
used in the engine will be returned to the fuel tank.
See Figure 29–1.
Fuel Pump
The electric fuel pump delivers fuel from the tank to
the system and develops fuel pressure. See Figure 29–2. The pump is usually located inside the fuel
tank, where it is cooled by the surrounding fuel.
Many pumps are incorporated with the fuel gauge
sender unit. Some earlier vehicles, usually imports,
mounted the pump outside the tank on the frame of
the underbody. In-tank pumps are replaced by removing the fuel tank or, in some cases, through an
access opening in the trunk floor.
The roller-cell pump, similar to power steering, is
driven by a permanent-magnet electric motor. It provides high-pressure fuel, at about 200 to 350 kPa (30
to 50 psi) for port injection and low-pressure fuel, at
about 70 kPa (10 psi) for throttle-body injection.
Some Fords use port style injectors in their central
fuel-injection (TBI type); system pressures are about
200 kPa (30 psi) for these units.
In the event of a restricted fuel filter or line
blockage, the pump is capable of producing pressures of 700 kPa (100 psi) or higher. An excess
pressure valve (relief valve) is built into the pump
Figure 29–1 Typical port fuel-injection system, indicating the location of various
components. Notice that the fuel pressure regulator is located on the fuel return side of
the system. The computer does not control fuel pressure, but does control the
operation of the electric fuel pump (on most systems) and the pulsing on and off of the
Figure 29–2 Schematic of a roller-cell fuel pump. Note #5 non-return valve, which prevents fuel pressure from bleeding
back through the pump, and #2 pressure limiter (relief) valve, which acts as a safety valve if the fuel filter or line is restricted.
(Courtesy Robert Bosch)
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Figure 29–3 The fuel filter is located between the fuel
pump and the fuel rail. Many filters are directional and have
an arrow (or different sized fittings) to prevent incorrect
installation. Most systems also have a filter “sock” at the
fuel tank pick-up. (Courtesy Robert Bosch)
as a safety feature. A one-way check valve is also
built into the pump outlet to prevent any fuel pressure in the lines or rail from bleeding back through
the pump.
Figure 29–4 The fuel rail is a hollow manifold that
supplies fuel to the injectors. Excess fuel not used in the
engine flows through a pressure regulator (usually
mounted on the rail) and returns to the tank. (Courtesy
DaimlerChrysler Corporation)
Fuel Filter
The fuel filter is a very important service item. See
Figure 29–3. It prevents any rust or dirt in the fuel
from reaching the fuel injectors where damage
would occur; injector blockage, sticking or leakage
are the usual result.
The filter is directional and may have an arrow
or different sized fittings to prevent mounting backwards. Filters are normally replaced after a given
number of kilometres or whenever major service is
performed, e.g., fuel pump replacement or injector
cleaning. Fuel system pressures must be released
and the fuel tank cap removed before loosening filter
lines. Pressurized fuel can spray for a long distance,
causing personal injury or a fire.
Fuel Rail (Port Injection)
The fuel rail (or ring) acts as a manifold supplying
fuel to each injector. See Figure 29–4. It also acts
as a mounting point for the fuel-pressure regulator. Some domestic vehicles have the regulator riveted to the fuel rail; both are supplied if either
needs replacing. The fuel injectors are sealed with
O rings where they mount to the rail; the O rings
should be replaced whenever the rail is removed
from the injectors.
Domestic fuel rails usually have a fuel-pressure
test fitting (Schrader valve) mounted on the rail,
which makes servicing much easier. Many imported
vehicles do not use Schrader valves and require special adaptors to test system pressures and fuel volume.
Fuel Injectors
Electronic fuel injectors are liquid-control solenoids
that open when electrically activated. See Figure
29–5. The injectors are pulsed on and off to control
fuel volume. The longer the injectors are held open,
the greater the amount of fuel injected into the manifold or intake port. Injectors are never operated at a
100% duty cycle. “On” time is called “pulse width”;
the longer the pulse width, the greater the fuel flow;
“on” time is usually in the 5 to 15 millisecond range.
Port Injection
Port injection systems used on gasoline-powered engines inject a fine mist of fuel into the intake manifold just above the intake valve. The pressure in the
intake manifold is below atmospheric pressure on a
running engine, and the manifold is therefore a vacuum. See Figure 29–6.
One major advantage of using port injection instead of the simpler throttle-body injection is that intake manifolds on port-injected engines only contain
air, not a mixture of air and fuel. No pre-heating of
the manifold is required to vaporize the fuel. This allows a cooler charge of intake air, which increases
power. Another advantage is the equal volume of
fuel provided to each cylinder. These “dry” manifolds
also allow the engine design engineer the opportunity to design long, tuned intake-manifold runners
that help the engine produce increased torque at
lower engine speeds.
Figure 29–5 Cross-section of a typical port fuel-injection nozzle assembly. These injectors are serviced as an assembly
only; no part replacement or service is possible other than cleaning or external O-ring replacement. Contamination at the
needle valve area is a common problem, especially with older type injectors. Later injectors, called deposit-resistant, changed
the tip design to reduce the formation of injector deposits.
Figure 29–6 A typical port-injection system squirts fuel
into the low pressure (vacuum) of the intake manifold,
about 75 mm (3 in.) from the intake valve. A buildup of
soft carbon on the intake valve often resulted from this
design. The petroleum industry responded in the mid1990s by increasing the percentage of detergent in
gasoline. The detergent also reduced injector
NOTE: Some port-injection systems used on engines
with four or more valves per cylinder may use two injectors per cylinder. One injector is used all the time,
and the second injector is operated by the computer
when high engine speed and high-load conditions are
detected by the computer. Typically, the second injector
injects fuel into the high-speed intake ports of the manifold. This system permits good low-speed power and
throttle response as well as superior high-speed power.
Fuel Pressure Regulator:
Port Injection
Fuel injectors that inject fuel into the intake port
are influenced by intake manifold vacuum. At idle,
manifold vacuum is very strong and a given
amount of fuel flows. As the throttle opens, manifold vacuum diminishes (pressure in the manifold
rises) and less fuel will flow because of the higher
pressure at the injector tip. Vacuum-modulated
pressure regulators increase fuel pressure about
35 kPa (5 psi) as the throttle is opened. See Figure
29–7. This compensates for the increase in manifold pressure.
Any excess fuel not injected into the engine returns to the tank via a return line.
The pressure regulator also prevents fuel pressure from bleeding into the return line when the engine is shut off; this maintains pressurized fuel at
the rail and injectors for faster starting.
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Figure 29–7 The vacuum-modulated pressure regulator
controls system fuel pressure with a spring-loaded
diaphragm. (a) Strong manifold vacuum (closed throttle)
works against the spring and fuel pressure decreases. (b) As
manifold vacuum drops (throttle opens), full spring pressure
is now exerted on the diaphragm and fuel pressure rises.
(Courtesy DaimlerChrysler Corporation)
Injector Firing Strategy
Fuel injectors have a number of operating strategies. They can all be fired at the same time (see Figure 29–8) with only one driver transistor. This is
known as simultaneous injection; it is not timed.
Some systems operate with two driver transistors;
half of the injectors fire on one revolution, the other
half fire on the second revolution; they also are not
timed. Sequential injection, which requires a separate ground wire and transistor for each injector
(ground side controlled), is timed. Injection usually
occurs at the end of the exhaust stroke as the intake valve is opening.
Figure 29–8 Wiring schematic
for a simultaneous injection
system. (Courtesy Toyota
Canada Inc.)
Sequential injection has a number of advantages over simultaneous injection: 1) Emissions
are reduced during low RPM and idle conditions.
2) It works with waste spark ignitions that fire the
spark plug every revolution. 3) OBD II systems
have the ability to cancel fuel delivery to any cylinder that is misfiring. This protects the catalytic
Many imported vehicles use a resistor, which reduces the voltage at the injectors to approximately
one-quarter of source voltage. This allows the use of
low resistance injectors, which improves injector response. Domestic vehicles operate the injectors at
source voltage and regulate injector response with
the PCM.
Overspeed protection is also built into most computer programs. If the engine is operated above red
line or is over-revved in neutral, the computer cuts
off every second injector (or similar strategy) to bring
the RPM down to a safe level.
Fuel Pump Electrical Circuits
The computer usually controls the operation of the
electric fuel pump, located in (or near) the fuel tank.
When the ignition switch is first turned on, the computer energizes the fuel pump relay and the pump
operates. See Figure 29–9. If the computer does not
receive a signal that the engine is rotating, the pump
will be shut off after 2 to 3 seconds. When the computer receives information that the engine is being
cranked, or has started, it continues to energize the
fuel pump. The signal may come from one or more of
the following:
Movement inside the vane airflow sensor from
air entering the engine.
Oil pressure is noted at the oil pressure sender.
An ignition tach signal (RPM) is present; this is
the most common.
Figure 29–9 Schematic of a Ford
fuel pump electrical circuit. After
2 to 3 seconds of pump operation,
the computer (ECA) must receive
an ignition signal (indicating the
engine is rotating) or it will shut
down the fuel pump relay. Note
the inertia safety switch in the
pump circuit. (Courtesy Ford
Motor Co.)
Figure 29–10 The inertia switch is used to shut off the electric fuel pump in case of an accident. Do not reset the switch
before checking for fuel leaks at the tank, lines, or engine compartment. (Courtesy Ford Motor Co.)
NOTE: This is a safety feature: if the engine stalls and
the tachometer (engine speed) signal is lost, the computer will shut off (de-energize) the fuel pump relay
and stop the fuel pump.
Inertia Safety Switch
Ford, Jaguar and Fiat use an inertia switch in the
fuel pump circuit to shut off the fuel pump in case of
an accident. See Figure 29–10. A permanent magnet
holds a steel ball in place; if an accident, or sharp impact, occurs, the steel ball breaks free and strikes a
target plate, which opens the switch contacts, shut-
ting off power to the pump. The switch is reset manually by depressing the reset button. Switch locations vary between vehicles; the switch may be in the
trunk, on the firewall or behind a kick panel. Check
the manual for location.
Throttle-body injection (TBI) is also known as Central Fuel Injection (CFI) or Single-Point Injection.
Throttle-body type of fuel injection uses one or two injectors (nozzles) to spray atomized fuel into the throttle body, which is similar to the base of a carburetor.
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Figure 29–11 Fuel is injected
above the throttle plate in this
CFI system. The pressure
regulator is not vacuummodulated, as intake manifold
vacuum does not have a major
influence on injection rates.
(Courtesy Ford Motor Co.)
Figure 29–12 Fuel delivery and return lines on this TBI system are similar to port fuel injection. (Courtesy General
Air and fuel mix in the throttle-body unit and flow as
a mixture down the intake manifold to the intake
valves. See Figure 29–11. The fuel pump, filter, and
lines are essentially the same as port injection. See
Figure 29–12. Because fuel is injected above the
throttle plate, intake manifold vacuum has no major
influence on the injector. Fuel pressure regulators are
not vacuum-modulated; fuel pressure is constant at
70 to 105 kPa (10 to 15 psi) depending on the model.
The ball-type tip of the TBI/CFI fuel injector is
much larger than the needle tips of port injectors
and it is prone to drip after the engine is shut off. See
Figure 29–13. Some TBI pressure regulators (GM,
Renault) have a bleed groove built into the pressureregulator valve seat to relieve fuel pressure after the
engine is turned off. Be aware of this condition when
testing residual fuel pressure; there will be no pressure remaining after a few seconds.
A typical TBI system uses a throttle-position (TP)
sensor and an idle air-control (IAC) valve. The TP is
an input to the computer and the IAC is an output
from the computer. The throttle-body injection unit
costs less to manufacture, because it only uses one or
two injectors (nozzles), whereas port-injection systems
require an injector for every cylinder plus the additional computer capabilities to control all the injectors.
Throttle body injection provides better driveability and fuel economy than a mechanical (or electronically controlled) carburetor, however all of the distribution and vaporization problems associated with
carburetted systems apply, as both air and fuel flow
through the manifold. Unlike a port-injection system, many TBI units require that heated air be used
with a heated intake manifold system to help vaporize the fuel that is injected into the incoming air inside the throttle-body unit.
Figure 29–13 A low-pressure TBI/CFI fuel injector feeds all cylinders
compared to a port fuel injector, which feeds only one cylinder. The
larger ball-type injector tip is prone to leak or drip when the engine has
been shut off. (Courtesy Ford Motor Co.)
Bosch continuous injection systems are also known as
K-Jetronic injection: K stands for konstant in German. They are found on many 1970s to 1990s European vehicles (i.e., Audi, BMW, Mercedes, Volkswagen
and Volvo, never on Asian or domestic automobiles).
Early CIS systems were mechanically operated;
there is no computer. See Figure 29–14. Later systems,
known as CIS-E, used a computer, a lambda (oxygen)
sensor and a frequency valve to trim fuel mixtures.
The frequency valve changes internal fuel pressures
inside the fuel distributor to vary the mixture.
Figure 29–14 Schematic of a Bosch CIS mechanical injection system. These units are found only on European vehicles.
(Courtesy Robert Bosch)
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
System Operation
Filtered fuel is pumped to the lower chamber of the
mixture control unit where it is regulated to about
500 kPa (75 psi) by the pressure regulator. Excess
fuel is returned to the tank.
Basic fuel control begins with an airflow sensor
plate mounted next to the mixture control unit. Air
entering the engine lifts the sensor plate; the
greater the flow of air, the higher the plate is lifted.
The arm on the airflow sensor plate contacts a fuel
control valve called a control plunger. As the sensor
plate lifts, it pushes on the control plunger, which
also lifts, increasing fuel delivery. See Figure 29–15.
Fuel flows from the mixture control unit to springloaded mechanical fuel injectors that open automatically when fuel pressure reaches 330 kPa (50 psi).
The General Motors CPI system is a combination of
a single electronic TBI-type injector and mechanical
spring-loaded fuel injectors. See the manifold design
in Chapter 9, Figure 9–42A.
The CPI fuel system is located inside a two-piece
split intake manifold. Fuel arriving at the CPI unit
is regulated by a built-in pressure regulator that
returns unused fuel to the tank. The single maxiinjector (computer activated) injects fuel into a base,
which contains six nylon tubes connected to six nylon fuel injectors (poppet nozzles). Fuel pressure at
the injectors overcomes spring tension and fuel is injected into the ports. See Figure 29–16.
Figure 29–15 Fuel delivery in a
Bosch CIS fuel distributor is metered
by a control plunger, which is lifted
by airflow at the sensor plate.
(Courtesy Robert Bosch)
Figure 29–16 Central port injection (CPI) operation. (Courtesy General Motors)
Later designs use separate injector solenoids
for each poppet valve, rather than a single maxiinjector. These systems are used primarily with V6 and V-8 light truck engines.
The most common injection system found from the
mid 1990s to date is returnless fuel injection. An
in-tank fuel pump module contains the pump, filter, pressure regulator, and fuel gauge, all in one
unit. See Figure 29–17. There is nothing outside
the tank, other than a single fuel line, rail, and injectors. See Figure 29–18. The extra computing
memory of the OBD II processor allows fuel volume
to be tailored to demand, regardless of changes in
manifold vacuum.
Removing the rear seat (or trunk mat) and servicehole cover allows access to the unit without removing the tank in most instances.
DaimlerChrysler was one of the first (mid-1990s)
to use returnless injection with their V-8 and V-10
engines. Since then it has been adopted by many domestic and import manufacturers and has become
the standard around the world.
Figure 29–17 The fuel pump, gauge, and pressure regulator are all mounted
inside the tank with returnless fuel injection. (Courtesy Toyota Canada Inc.)
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Figure 29–18 Fuel rails with returnless fuel
injection contain an inlet fitting and
pressure-gauge port. There is no return
line. Pressure remains constant at 275 kPa
(40 psi). (Courtesy DaimlerChrysler
A few Asian manufacturers—Mitsubishi, Toyota,
and Isuzu—are using gasoline direct injection (GDI)
with selected models. GDI sprays high-pressure fuel
(8000 to 13 000 kPa, 1200 to 1950 psi) into the combustion chamber as the piston approaches the top of
the compression stroke. See Figure 29–19.
The combination of high-pressure swirl injectors, with almost instant vaporization, and modified
combustion chamber and port design allows the engine to run with a much leaner air/fuel mixture than
conventional intake port injection. Fuel economy has
shown a major improvement and engine emissions
have been reduced.
Lean-burn engines traditionally lower hydrocarbon (HC) and carbon monoxide (CO) emissions; however, oxides of nitrogen (NOx) emissions rise because
of the elevated combustion temperatures created by
lean mixtures. Increasing the amount of exhaust gas
(EGR) fed into the incoming air and a special catalytic converter reduces NOx to a very low level. It is
expected that GDI engines will become more common as emission and fuel economy standards become
more stringent.
Most electronic fuel-injection computer systems use the
ignition primary (pickup coil or crank sensor) pulse as the
trigger for when to inject (squirt) fuel from the injectors
(nozzles). If this signal were not present, no fuel would be
injected. Because this pulse is also necessary to trigger
the module to create a spark from the coil, it can be said
that “no spark” could also mean “no squirt.” Therefore, if
the cause of a no-start condition is observed to be a lack
of fuel injection, do not start testing or replacing fuelsystem components until the ignition system is checked
for proper operation.
Check the air filter and replace as needed.
Check the air induction system for obstructions.
Check the condition of all vacuum hoses. Replace
any hose that is split, soft (mushy), or brittle. Be
sure to use the correct type of hose designed for
use on a vacuum system. Using fuel line hose
instead of vacuum hose can cause the hose to be
sucked closed, creating more problems. This is
especially true for the PCV valve hose.
Check the positive crankcase ventilation (PCV)
valve for proper operation or replacement as
NOTE: The use of an incorrect PCV valve can cause a
rough idle or stalling.
No Spark, No Squirt
Diagnosing Electronic
Fuel-Injection Problems
Using Visual Inspection
All fuel-injection systems require the proper amount
of clean fuel delivered to the system at the proper
pressure and the correct amount of filtered air. The
following items should be carefully inspected before
proceeding to more detailed tests.
Check all fuel-injection electrical connections for
corrosion or damage.
Check for gasoline at the vacuum port of the fuel
pressure regulator if the vehicle is so equipped.
Gasoline in the vacuum hose at the fuel pressure
Figure 29–19 Gasoline direct injection (GDI). Note the high-pressure swirl fuel injector at the combustion chamber.
(Courtesy Toyota Canada Inc.)
The Ear Test
No, this is not a test of your hearing, but rather using your
ear to check that the electric fuel pump is operating. The
electric fuel pump inside the fuel tank is often difficult to
hear running, especially in a noisy shop environment. A
commonly used trick to better hear the pump is to use a
funnel in the fuel filter neck.
regulator indicates that the regulator is defective
and requires replacement.
Test Connectors
Many vehicles have test procedures that allow the
technician to operate the electric fuel pump without
starting the engine; these vary between makes, but
the following is typical:
Open the meter plate at the vane airflow sensor.
See Figure 29–20.
Jumper two test terminals at the airflow sensor.
Jumper specified terminals at the fuel pump relay.
Ground the fuel-pump test connector (activates
the relay).
Figure 29–20 The vane airflow meter plate should open
with light pressure to the fully open position and return to
rest without dragging or binding. Many European and Asian
vehicles (to mid-1990s) also incorporate fuel-pump safety
contacts; opening the plate with a finger (engine key “on”) will
activate the fuel pump. Domestic vehicles with this type of
meter use a tach signal, instead of contacts, for pump control.
(Courtesy Robert Bosch)
Power the test connector (powers the fuel pump).
Activate the fuel pump relay with a scan tool.
Follow the manufacturer’s instructions exactly; a
wrong connection could ruin the computer, wiring or
relay. See Figure 29–21.
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
The Quad Four Story
Figure 29–21 Most General Motors fuel-injected
vehicles are equipped with a fuel pump test connector.
The operation of the fuel pump can be checked by
connecting a 12 volt test light to the positive ()
terminal of the battery and the point of the test light to
the test connector. Turn the ignition to on (engine off).
The light should either go out or come on for 2 seconds.
This is a simple test to check to see if the computer can
control the fuel pump relay.
Port Fuel-Injection
System Diagnosis
To determine if a port fuel-injection system, including the fuel pump, injectors, and fuel pressure regulator, are operating okay, follow these steps:
1. Attach a fuel pressure gauge to the Schrader
valve on the fuel rail.
NOTE: Some fuel rails may not have a Schrader
valve on the rail and therefore, special adapters may
be required.
2. Turn the ignition key on or start the engine to
build up the fuel pump pressure (it should be
about 210 to 350 kPa [30 to 50 psi]).
3. Wait 20 minutes and observe the fuel pressure
retained in the fuel rail and note the value.
(The fuel pressure should not drop more than
140 kPa [20 psi] in 20 minutes.) If the drop is
less than 140 kPa (20 psi) in 20 minutes,
everything is okay. If the drop is greater than
140 kPa (20 psi) in 20 minutes, there is a
possible problem with:
• The check valve in the fuel pump
• Leaking injectors
• A defective (leaking) fuel pressure
To determine which unit is defective, perform
the following with the gauges still connected:
A service technician was diagnosing a rough-running condition on a General Motors Quad Four engine. The paper test indicated a cylinder miss. To help determine
which cylinder was possibly causing the problem, the
technician disconnected the fuel-injector connectors one
at a time. When the injector was disconnected from
cylinder #2, the engine did not change in the way it was
running. A compression test indicated that the cylinder
had good compression. The technician removed the ignition cover and used conventional spark plug wires to
connect the coils to the spark plugs. The technician then
connected short lengths of rubber vacuum hose to each
of the plugs. The technician then touched each rubber
hose with a grounded test light to ground out each cylinder. Again, cylinder #2 was found to be completely dead.
Then the technician made a mistake by assuming
that the fault had to be a defective fuel injector. A replacement fuel injector did not solve the problem. Further testing of the injectors revealed that injector #3
was shorted. Because both injectors #2 and #3 share
the same driver inside the computer, the injector that
was shorted electrically required more current than the
normal good injector. Because the computer driver circuit controls and limits current flow, the defective
(shorted) injector would fire (squirt), whereas the good
injector did not have enough current to work.
CAUTION: The use of fuel-injector cleaner may damage
the electrical windings of the fuel injector. Gasoline flows over
the copper coil windings of an injector to help keep it cool. If
a strong solvent is used in the fuel-injection cleaner,the varnish
insulation on the coil may be damaged. As a result, the coil
windings may short against each other, lowering the resistance
of the injector.
• Re-energize the electric fuel pump for
10 seconds.
• Clamp the fuel supply line, wait 10 minutes
(see Caution box on the next page). If the
pressure drop does not occur, replace the fuel
pump. If the pressure drop still occurs,
continue with the next step.
• Repeat the pressure build-up of the electric
pump and clamp the fuel return line. If the
pressure drop time is now okay, replace the
fuel pressure regulator.
• If the pressure drop still occurs, one or more of
the injectors is leaking. Remove the injectors
with the fuel rail and hold over paper. Replace
those injectors that drip one or more drops
after 10 minutes with pressurized fuel.
The Electric Fuel Pump Clue
The on-board computer controls the operation of the
electric fuel pump, fuel-injection pulses, and ignition timing. With a distributorless ignition system, it is difficult at
times to know what part in the system is not operating if
there is no spark from any of the ignition coils. A fastand-easy method for determining if the crankshaft sensor
is operating is to observe the operation of the electric
fuel pump. In most electronic fuel-injection systems, the
computer will operate the electric fuel pump for only a
short time (usually about 2 seconds) unless a crank pulse
is received by the computer.
Most manufacturers provide a fuel pump test lead
with which the technician can monitor the electrical operation of the pump. On most vehicles, if voltage is maintained to the pump during engine cranking for longer
than 2 seconds, then the crankshaft sensor is working. If
the pump only runs for 2 seconds then turns off during
cranking of the engine, the crankshaft sensor, wiring, or
computer may be defective.
NOTE: Another way of testing is to use a scan tool. If an
RPM signal is processed and displayed by the computer, then
the crank sensor is functioning.
The Rich-Running Chrysler
A four-cylinder Chrysler was running so rich that black
smoke poured from the exhaust all the time. It was
equipped with a TBI-type fuel-injector system, and the
fuel pressure was fixed at about 260 kPa (38 psi)—the
same as the maximum fuel-pump pressure. A replacement fuel-pressure regulator did not correct the higherthan-normal fuel pressure. The fuel return line was also
carefully inspected for a kink or other obstruction that
may have caused excessive fuel pressure. The technician
discovered the root cause of the problem to be a stuck
shuttle valve, a part of many Chrysler TBI systems used
to close off the fuel return to the tank to keep the pressure high, permitting faster restarts when the engine is
hot. The shuttle valve simply slides downward on an incline to close off the fuel regulator return passage. The
technician removed the shuttle valve and cleaned it. Vehicle operation then returned to normal and both the
technician and the customer were satisfied that a low
cost and fast solution was found.
CAUTION: Do not clamp plastic fuel lines. Connect
shut-off valves to the fuel system to shut off supply and
return lines.
The most common gasoline fuel injection systems operate with system pressures ranging from 70 kPa (10
psi) on low pressure TBI/CFI to 350 kPa (50 psi) on
port injection. There are exceptions, so service specifications should always be checked before starting.
Typical System Pressures
kPa (psi)
Pump Pressure
kPa (psi)
TBI units
70 kPa (10 psi)
140 kPa (20 psi)
High- pressure
TBI units
210 kPa (30 psi)
450 kPa (65 psi)
Port fuel-injection
350 kPa (50 psi)
700 kPa (100 psi)
Central port
fuel injection
420 kPa (60 psi)
700 kPa (100 psi)
Bosch K-Jetronic
525 kPa (75 psi)
700 kPa (100 psi)
280 kPa (40 psi)
550 kPa (80 psi)
Maximum fuel pressure should never be
reached provided the fuel pressure regulator is operating and there is no blockage in the filter or lines;
blockage before the gauge test fitting may not show
a pressure rise at the gauge.
Closed loop injection returns excess fuel to the
tank. The continuous flow of fuel cools the injector
and helps prevent vapour from forming in the fuel
system. Although vapour or foaming in a fuel system
can affect engine operation, the cooling and lubricating flow of the fuel helps to ensure the durability
of the injector nozzles.
Returnless injection systems cycle any excess fuel
at the regulator inside the tank. The fuel is not exposed to high underhood temperatures (until it is
used at the injectors) or heated by pumping it through
the rail and back to the tank; the fuel remains cool.
To measure fuel-pump pressure, locate the
Schrader valve, if equipped, or install a suitable adaptor. Attach a fuel pressure gauge as shown in Figure
29–22. Check the pressure while the engine idles. The
fuel pressure should remain constant on all systems
other than vacuum modulated port fuel injection where
pressures vary with changes in manifold vacuum.
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Figure 29–22 A fuel pressure gauge connected to the fuel
pressure tap (Schrader valve) on a port-injected V-6 engine.
Port Fuel-Injection Pressure
Regulator Diagnosis
Most port fuel-injected engines use a vacuum hose
connected to the fuel pressure regulator. At idle, the
pressure inside the intake manifold is low (high vacuum). Intake manifold vacuum is applied above the
diaphragm inside the fuel pressure regulator. This
reduces the pressure exerted on the diaphragm and
results in a drop (about 35 kPa or 5 psi) in fuel pressure applied to the injectors. To test a vacuumcontrolled fuel pressure regulator, follow these steps:
1. Connect a fuel pressure gauge to monitor the
fuel pressure.
2. Locate the fuel pressure regulator and
disconnect the vacuum hose from the regulator.
NOTE: If gasoline drips out of the vacuum hose when
removed from the fuel pressure regulator, the regulator
is defective and will require replacement.
3. Using a hand-operated vacuum pump, apply
vacuum, about 500 mm (20 in.) Hg to the
regulator. The regulator should hold vacuum. If
the vacuum drops, replace the fuel pressure
regulator. See Figure 29–23.
4. With the engine running at idle speed, reconnect
the vacuum hose to the fuel pressure regulator
while watching the fuel pressure gauge. The fuel
pressure should drop (about 35 kPa, 5 psi) when
the hose is reattached to the regulator.
Testing Fuel-Pump Volume
Fuel pressure alone is not enough for proper engine
operation. Sufficient fuel capacity (flow) must be at
least 1 litre (2 pints) per minute (0.5 litre or 1 pint
in 30 seconds).
Figure 29–23 If the vacuum hose is removed from the fuel
pressure regulator when the engine is running, the fuel
pressure should increase. If it does not increase, then the fuel
pump is not capable of supplying adequate pressure or the
fuel pressure regulator is defective. If gasoline is visible in the
vacuum hose, the regulator is leaking and should be replaced.
All fuel must be filtered to prevent dirt and impurities from damaging the fuel-system components
and/or engine. The first filter (sock) is inside the gas
tank and is usually attached to the fuel pump (if the
pump is electric) and/or fuel-gauge sending unit. The
main fuel filter is usually located between the fuel
tank and the fuel rail or inlet to the fuel-injection
system. For long engine and fuel-system life and optimum performance, the main fuel filter should be
replaced every year or every 24 000 km (15 000 mi).
Consult vehicle manufacturers’ recommendations
for exact time and kilometre (mileage) intervals.
If the fuel filter becomes partially clogged, the
following are likely to occur:
1. There will be low power at higher engine
speeds. The vehicle usually will not go faster
than a certain speed (engine acts as if it has a
built-in speed governor).
Stethoscope Fuel Injection Test
A commonly used test for injector operation is to listen
to the injector with a stethoscope while the engine is operating at idle speed. See Figure 29–24. All injectors
should produce the same clicking sound. If any injector
sounds different from the others, further testing or replacement may be necessary. All injectors should make a
“clicking” sound. If any injector makes a “clunking” or
“rattling” sound, it should be tested further or replaced.
With the engine still running, place the end of the stethoscope probe to the return line from the fuel pressure regulator. See Figure 29–25. The sound of fuel should be
heard flowing back to the fuel tank. If no sound of fuel is
heard, then the fuel pump, fuel filter, or the fuel pressure
regulator is at fault.
2. The engine will cut out or miss on acceleration,
especially when climbing hills or during heavyload acceleration.
A weak or defective fuel pump can also be the cause
of the symptoms just listed. If an electric fuel pump for
a fuel-injected engine becomes weak, the engine may
also be hard to start, or it will idle rough or stall.
CAUTION: Be certain to consult the vehicle manufacturer’s recommended service and testing procedures
before attempting to test or replace any component of a
high-pressure electronic fuel-injection system.
NOTE: Most electric fuel pumps have a life expectancy of about 160 000 km (100 000 mi) before replacement. The usual cause of failure is brush wear at
the commutator. Some manufacturers are now using
brushless, permanent magnet fuel pumps, which provide a major improvement in service life.
The arcing of the electric current from the fuel pump
brushes to the armature commutator will not cause a
gasoline fire or explosion, as there is insufficient oxygen
in the pump while it is mounted on the vehicle.
This is not true if the pump has been removed from
the vehicle; any remaining fuel vapours will mix with air if
the pump is electrically activated (tested) off the vehicle.
The pump could explode! Always follow the manufacturers’ procedures when testing pumps.
Figure 29–24 All fuel injectors should make the same
sound with the engine running at idle speed. A lack of
sound indicates a possible electrically open injector or a
break in the wiring. A defective computer could also be the
cause of a lack of clicking (pulsing) of the injectors.
Figure 29–25 Fuel should be heard returning to the fuel
tank at the fuel return line if the fuel pump and fuel
pressure regulator are functioning correctly.
Fuel-system pressure is controlled by a fuel pressure regulator at the fuel rail or throttle body. A restricted fuel
filter or line will cause fuel pressure to increase, up to 700
kPa (100 psi) in some cases. The fuel pump slows down
because of the added load and usually becomes noisier. A
complaint of “whining noise in the rear” could be corrected by replacing the fuel filter. A fuel volume test (after the filter) will verify the diagnosis.
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
NOTE: The term noid is simply an abbreviation of the
word solenoid. Injectors use a movable iron core and
are therefore a solenoid. Therefore, a noid light is a replacement for the solenoid (injector).
Possible noid light problems and causes include
the following:
1. The light is off and does not flash. The
problem is an open in either the power side or
ground side (or both) of the injector circuit.
2. The noid light flashes dimly. A dim noid light
indicates excessive resistance or low voltage
available to the injector. Both the power and
ground side must be checked.
3. The noid light is on and does not flash. If
the noid light is on, then both a power and a
ground are present. Because the light does not
flash (blink) when the engine is being cranked or
started, then a short-to-ground fault exists
either in the computer itself or in the wiring
between the injector and the computer.
Checking Fuel-Injector Resistance
Figure 29–26 (a) Noid lights are usually purchased as an
assortment so that one is available for any type or size of
injector wiring connector. (b) The connector is unplugged
from the injector and a noid light is plugged into the
injector connector. The noid light should flash when the
engine is being cranked if the power circuit and the pulsing
to ground by the computer are functioning correctly.
Each port fuel injector must deliver an equal
amount of fuel or the engine will idle rough or perform poorly.
The electrical balance test involves measuring
the injector coil-winding resistance. For best engine
operation, all injectors should have the same electrical resistance. To measure the resistance, carefully
release the locking feature of the connector and remove the connector from the injector.
Always check the service information for the exact specifications for the vehicle being checked.
NOTE: Some engines require specific procedures to
gain access to the injectors. Always follow the manufacturers’ recommended procedures.
Testing for an Injector Pulse
One of the first checks that should be performed
when diagnosing a no-start condition is whether
the fuel injectors are being pulsed by the computer. Checking for proper pulsing of the injector
is also important in diagnosing a weak or dead
A noid light is designed to electrically replace
the injector in the circuit and to flash if the injector
circuit is working correctly. See Figure 29–26. To
use a noid light, disconnect the electrical connector
at the fuel injector and plug the noid light into the
injector harness connections. Crank or start the engine. The noid light should flash regularly.
With an ohmmeter, measure the resistance
across the injector terminals. Be sure to use the lowohms feature of the digital ohmmeter to be able to
read in tenths (0.1) of an ohm. See Figures 29–27 and
29–28. Subtract the lowest reading injector from the
highest. For example,
Highest-resistance injector 17.4 ohms
Lowest-resistance injector 17.2 ohms
Difference 0.2 ohms
Acceptable maximum differences should be
limited to 0.3 to 0.4 ohms. A greater difference in
resistance indicates a possible problem. Further
testing should be performed. The resistance of the
Figure 29–27 Connections and settings necessary to measure fuel-injector resistance.
(Courtesy of Fluke Corporation)
injectors should be measured twice—once when
the engine (and injectors) are cold and once after
the engine has reached normal operating temperature. If any injector measures close to or over
1.0 ohm different from the others, it must be replaced after making certain that the terminals of
the injector are electrically sound.
Measuring Resistance
of Grouped Injectors
Figure 29–28 To measure fuel-injector resistance, a
technician constructed a short wiring harness with a
double banana plug that fits into the V and COM terminals
of the meter and an injector connector at the other end.
This setup makes checking resistance of fuel injectors
quick and easy.
Many vehicles are equipped with a port fuelinjection system that fires two or more injectors at
a time. For example, a V-6 may group all three injectors on one bank to pulse on at the same time,
then the other three injectors will be pulsed on.
This sequence alternates. To measure the resistance of these injectors, it is often easiest to measure each group of three that is wired in parallel.
The resistance of three injectors wired in parallel
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
tance. If both groups measure 4 ohms, then it is
likely that all six injectors are okay. However, if one
group measures only 2.9 ohms and the other group
measures 4 ohms, then it is likely that one or more
fuel injectors are defective (shorted). This means
that the technician now has reasonable cause to remove the intake manifold to get access to each injector for further testing. See Figure 29–29.
Pressure-Drop Balance Test
Figure 29–29 The fuel injector wiring connector on this
General Motors 3.1-litre V-6 is hidden and attached to the
rear of the intake manifold. Both groups of three injectors
can be easily measured using an ohmmeter. Both groups of
injectors should measure within 0.5 ohm of each other.
is one-third of the resistance of each individual injector. For example,
Injector resistance 12 ohms
Three injectors in parallel 4 ohms
A V-6 has two groups of three injectors. Therefore, both groups should measure the same resis-
The pressure balance test involves using an electrical timing device to pulse the fuel injectors on for a
given amount of time (usually 500 milliseconds)
and observing the drop in pressure that accompanies the pulse. If the fuel flow through each injector
is equal, the drop in pressure in the system will be
equal. Most manufacturers recommend that the
pressures be within about 10 kPa (1.5 psi) of each
other for satisfactory engine performance. This test
method not only tests the electrical functioning of
the injector (for definite time and current pulse) but
also tests for mechanical defects that could affect
fuel flow.
Scope Testing Fuel Injectors
A scope such as a digital storage oscilloscope (DSO)
can be attached to the pulse side of the injector and
the waveform checked and compared to a knowngood pattern. See Figures 29–30 and 29–31.
Figure 29–30 The injector on-time is called the pulse width. (Courtesy of Fluke
Figure 29–31 A typical peak and hold
fuel-injector waveform. Most fuel
injectors that measure less than 6 ohms
will display a similar waveform.
(Courtesy of Fluke Corporation)
Figure 29–32 Fuel-injector cleaner is fed into the fuel system with this
cleaning unit; shop air is attached to the regulator fitting. Aerosol cans,
already pressurized and containing pre-mixed cleaner, are also used; however,
they contain less cleaner and are often more expensive. (Courtesy OTC
Division, SPX Corporation)
Most fuel injectors can be cleaned on the vehicle by
feeding injector-cleaning liquid into the fuel rail, or
TBI/CFI test port while the engine is running.
One common piece of equipment is shown in
Figure 29–32. Liquid cleaner, which may require
diluting with gasoline, is poured into the container
after the top has been unscrewed. The top, containing an adjustable air pressure regulator, is reinstalled and a shop air hose is attached to the
regulator. Ensure that the shut-off valve is closed
and adjust the container pressure to 35 kPa (5 psi)
lower than the fuel-injection-system operating
pressure. TBI/CFI systems operate with low pressures; a 15 kPa (2 psi) lower setting is fine with
these units.
Hang the cleaning unit under the hood and attach the supply hose to the Schrader valve (or adaptor) on the fuel rail or as directed in the operating instructions. Disconnect the wiring to the electric fuel
pump on the vehicle. Block the fuel return line by
clamping, if rubber, or by installing a shut-off valve
if plastic or plastic-lined. See Figure 29–33.
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Figure 29–33 Typical hookup for on-vehicle injector cleaning. Note the blocked fuel return line
and the unplugged wiring to the fuel pump. (Courtesy OTC Division, SPX Corporation)
Frequently Asked Question
If Three Out of Six Injectors
Are Defective, Should I Also Replace
the Other Three?
This is a good question. Many service technicians recommend that the three good injectors also be replaced
along with the other three that tested as being defective.
The reasons given by these technicians include:
• All six injectors have been operating under the same
fuel, engine, and weather conditions.
• The labour required to replace all six is just about the
same as replacing only the three defective injectors.
• Replacing all six at the same time helps ensure that
Figure 29–34 A set of six new injectors.
all of the injectors are flowing the same amount of
fuel so that the engine is operating most efficiently.
With these ideas in mind, the customer should be informed and offered the choice. Complete sets of injectors such as those in Figure 29–34 can be purchased at a
reasonable cost.
Remember always to keep a fire extinguisher,
(suitable for gasoline) on hand whenever working
with fuel injection.
Open the shut-off valve, start the engine and let
it run until the container runs out of fluid. Some manufacturers recommend a fast idle only; others run the
engine at various speeds. Remove the equipment, reconnect the pump, remove the return line shut-off,
restart the engine and check the injector operation.
Cleaning the injectors on the vehicle will usually
correct leaking or contamination at the injector tip;
if this operation is not successful, the injectors must
be removed for electronic cleaning (high frequency
vibration) or replacement.
On an engine equipped with fuel injection (TBI or
port injection), the idle speed is controlled by increasing or decreasing the amount of air bypassing
the throttle plate. Again, an electronic stepper motor
is used to maintain the correct idle speed. This control is often called the idle air control (IAC). See
Figures 29–35 through 29–37.
When the engine stops, most IAC units will extend the conical valve until the valve bottoms in
the air bypass passage. The computer notes this
position and then moves the valve outward to get
Figure 29–36 A typical IAC.
Figure 29–35 An idle air control (IAC) controls idle
speed by controlling the amount of air that passes around
the throttle plate. More airflow results in a higher idle
speed. (Courtesy of Fluke Corporation)
Figure 29–37 Some idle air control units are purchased
with the housing as shown. Carbon buildup in these
passages can cause a rough or unstable idling or stalling.
ready for the next engine start. When the engine
starts, the engine speed is high to provide for
proper operation when the engine is cold. Then, as
the engine gets warmer, the computer reduces engine idle speed gradually by reducing the number
of counts or steps commanded by the IAC.
When the engine is warm and restarted, the idle
speed should momentarily increase, then decrease to
normal idle speed. This increase and then decrease in
engine speed is often called an engine flare. If the engine speed does not flare, then the IAC may not be
working (it may be stuck in one position).
Some air control valves (Ford, Hitachi) can be removed and disassembled for cleaning. Never use liquid cleaners on electrical components or plastic control valves as damage can occur.
Throttle Body Icing
Port fuel injection manifolds are not heated; air only
passes through the runners. Under certain low temperature, high humidity conditions, moisture in the
incoming air will freeze at the throttle plate area of
the throttle body. Many current throttle bodies incorporate a pocket, or passage, for engine coolant to
warm the body. See Figure 29–38.
Electronic Throttle Control
Most electronic throttle control systems do not use a
throttle cable. An electric motor on the side of the
throttle body operates the throttle plate when commanded by the PCM. An accelerator position sensor at
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Figure 29–38 This electronic throttle body uses engine coolant to prevent throttle plate icing. Note the location of the
throttle control motor and position sensor. (Courtesy Toyota Canada Inc.)
the accelerator pedal sends a signal to the PCM, which
in turn, adjust the throttle motor to match the driver’s
input. The throttle position sensor on the throttle body
sends throttle angle information to the PCM.
Electronic throttles originated with traction
control systems where the computer reduces throttle opening when wheel spin is detected. Since
then, it has become common with or without traction control.
Conditions of excessive RPM or engine overheating may also trigger reduced throttle opening.
Throttle Plate Contamination
The positive crankcase ventilation (PCV) system
picks up ventilating air, usually between the mass
airflow sensor and the throttle plate. See Figure
29–39. Crankcase fumes often backfeed into the
throttle body causing a buildup of deposits at the
throttle plate and bore. These deposits are normally
removed during regular maintenance service or
when a driveability concern is noted.
The throttle plates of a port fuel-injected engine
may require cleaning, especially if the following conditions exist:
Rough idle
Surging at idle
Hesitation during acceleration
Higher than normal IAC counts as displayed on
a scan tool.
See Figures 29–40 and 29–41.
Speed density fuel injection relies on information
typically from MAP, CTS, ACT, RPM, and TPS for calculating fuel delivery. An air leak in the hose between the air cleaner and the throttle body usually
will not affect driveability.
The opposite is true with mass-air systems; any
air leaks could change the mass airflow sensor reading and cause hard starting and rough running. This
usually occurs during open loop operation when fuel
is not being trimmed by the oxygen sensor. See Figures 29–42 and 29–43.
Diesel injection systems have seen many changes
over the past few years, driven in part by new, more
stringent emissions regulations and a call for increased economy. Earlier systems used a mechanical
fuel injection pump to meter fuel delivery; however,
Figure 29–39 Airflow through the positive crankcase ventilation (PCV) system.
Note the closure hose at the front cam cover; blow-by gases may back-flow into
the air intake under certain driving conditions, i.e., full-throttle, high RPM
operation. (Courtesy Toyota Canada Inc.)
Figure 29–40 (a) Dirty throttle plate. This throttle plate was so dirty that the technician removed the entire throttle body
to be sure it was thoroughly cleaned. (b) Most throttle plates can be cleaned on the vehicle using a brush and throttle body
cleaner. Be sure the cleaner is safe for oxygen sensors.
Figure 29–41 Some vehicles, such as this Ford, have
labels on the throttle body warning not to clean the
throttle plates. A slippery coating is placed on the throttle
plate and throttle bore that prevents deposits from
sticking. Cleaning this type of housing can remove this
protective coating.
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Figure 29–42 Schematic of a General Motors 2.8 litre gasoline fuel injection system with a mass airflow sensor.
Many European and Asian vehicles use a cold-start fuel injector (as does this vehicle); however, cold-start injectors
are not common in domestic vehicles, which typically use a major increase in injector pulse-width (“on” time) or a
primer pulse (extra injector pulse) for cold engine starting. (Courtesy General Motors)
this did not allow the precise control required to
meet new standards. Electronic systems were introduced in the mid to late 1990s. We will start with
conventional fuel injection.
Conventional fuel injection uses, for the most part, all
mechanical components. There is limited electrical use.
Other than glow-plug circuits, solenoids, block heaters,
and fuel heaters, fuel delivery is governed by a mechanical injection pump. See Figure 29–44. Although
here are variations between makes and engine types,
the following is common with most systems.
Fuel tanks—Very similar to gasoline vehicles;
multiple tanks are often used for long distance
vehicles such as vans or pick-up trucks. The fuel
supply line in the tank usually contains a prefilter to limit large contaminants from entering
the system.
Lift pump—Transfers fuel from the fuel tank,
through the fuel filter and on to the delivery
system. This may be an electric pump or a
mechanical pump driven by the engine.
Fuel filter—Very important with a diesel engine
as any small particles or abrasives that get past
the filter may cause damage to the injection
pump or injectors. See Figure 29–45. Hand
priming pumps are often found on the fuel filter;
they are used to remove trapped air from the fuel
system and to force fuel to the injection pump.
Many late-model systems remove air
Water/fuel separators—Water in the fuel creates
a number of driveability problems as well as
system damage. Water is heavier than diesel fuel
and will accumulate at the bottom of the
separator, where it is drained as part of regular
maintenance. Some separators have a sensor
that illuminates a warning light on the
instrument panel when the water reaches a
given level. See Figure 29–46. Many late-model
systems incorporate the fuel filter, water
separator, and fuel heater in one unit.
Figure 29–43 The Bosch L-Jetronic (L stands for luft, which is “air” in German) gasoline fuel injection system. The
vane airflow sensor measures airflow, not mass. It is not as accurate as a mass air sensor, but it is a major improvement
over speed-density systems. L-Jetronic injections are used on many European, Asian and some domestic vehicles from
the mid-1970s to the mid-1990s. (Courtesy Robert Bosch)
Fuel heaters—Because diesel fuel has a tendency
to wax and thicken when cold, electric heaters
are often used to warm the fuel. Canadian diesel
fuels are also blended to match seasonal
temperatures; a very light fuel is supplied for
winter use.
Fuel injection pump—Diesel fuel must be
injected into the combustion chamber area at
extremely high pressure, over 17 500 kPa (2500
psi), to overcome cylinder pressures.
An injection pump increases fuel pressure, controls speed and power by metering the volume of fuel
injected, and directs the fuel to the correct injector. It
may also contain a governor, which limits the maximum RPM of the engine, and a fuel shut-off.
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Figure 29–44 Schematic of the fuel delivery and return on a conventional (mechanical) diesel fuel injection. (Courtesy
Ford Motor Co.)
Figure 29–45 A diesel fuel filter with built-in priming
pump. (Courtesy Ford Motor Co.)
Figure 29–46 A water/fuel separator with a water level
warning light. (Courtesy Ford Motor Co.)
Two types of mechanical injection pumps are common
with conventional systems: the in-line and the rotary.
pressure fuel opens the delivery valve spring, which allows fuel to move through steel lines to the injectors,
where it is supplied to the engine. See Figure 29–49.
In-line Injection Pumps (4-Cycle)
In-line pumps are usually found on large trucks and
older passenger car/light truck applications. See Figure
29–47.The pump is driven at one-half the engine speed,
which means the injection-pump camshaft makes one
complete revolution for each two turns of the engine.
When the pump cam lobe pushes up on the cam follower
and plunger, the fuel above the plunger is put under
very high pressure. See Figure 29–48. This high-
Never check for fuel leaks by running
your hand over the lines—a high pressure
leak could penetrate your skin, enter the
blood stream and cause poisoning.
It is good practice, instead, to move a piece of light
coloured cardboard along the lines, checking visually for
signs of liquid fuel on the cardboard. See Figure 29–50.
Figure 29–47 An in-line
diesel injection pump.
(Courtesy Ford Motor Co.)
Figure 29–48 A diesel fuel injection pump: start of injection. (Courtesy Ford Motor Co.)
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Figure 29–50 Diesel fuel leak testing. Use cardboard to test
for leaks, NEVER your hand! (Courtesy Ford Motor Co.)
Figure 29–49 Fuel passing through the delivery valve to
the injector. (Courtesy Ford Motor Co.)
Figure 29–51 Movement of the
control rack in an in-line injection
pump changes the fuel delivery
volume. (Courtesy Robert Bosch)
Fuel Control—In-line Injection
Remember that diesels do not use a throttle plate;
under most operating conditions the engine takes in
far more air than it requires. Power and speed are
controlled by the amount of fuel injected; more fuel
equals higher speed and greater power.
Note the control rack in Figure 29–51, which is
connected to the accelerator pedal. As the rack is
moved in or out, it rotates a gear and control sleeve,
which turns the plunger.
A tapered groove, called the helix, is machined
into the plunger. This increases or decreases the
amount of fuel as the plunger is rotated. The helix
controls fuel volume by opening or restricting a passage to the spill port: a large opening means less fuel
is left in the barrel for injection, a restricted opening
leaves more fuel in the barrel and a greater volume
of fuel is injected. See Figure 29–52.
Governors are usually incorporated into the fuel injection pump where engine speed is controlled by
limiting the amount of fuel supplied to the injectors.
The most common type of in-line pump governor
uses flyweights, which are held in by spring pressure. See Figure 29–53. At higher RPM, centrifugal
force causes the flyweights to move outward against
the spring; this movement limits fuel-rack travel,
which in turn restricts fuel delivery and prevents engine over-revving.
Runaway Engines
A sticking governor, in extreme cases, may continue to
supply fuel to the engine. This allows the RPM to build
until the engine destroys itself. In order to stop the engine, turn off the fuel line shut-off valve (if equipped) or
stuff rags into the air cleaner intake to shut off the air.
Diesel engine manufacturers generally caution
against running the engine with the air intake hose disconnected from the intake manifold; not only could this
allow dirt and foreign material to enter the engine, serious personal injury could result if a body part is pulled
into the opening.
Figure 29–52 The helix controls fuel volume by varying
the opening to the spill port. (Courtesy Ford Motor Co.)
Figure 29–53 Governors control engine speed by limiting fuel at higher RPM. (Courtesy Ford Motor Co.)
The common rotary pump, often called a distributor
pump, uses a rotating motion rather than the reciprocating action of the in-line pump. Not only is the
pressure of the incoming fuel raised, a controlled volume of fuel is sent to the proper cylinder. This type of
pump normally contains a fuel metering valve, governor and a mechanical or electric fuel shut-off. See
Figure 29–54.
Fuel enters the pump through a centre port, flows to
a metering valve (controlled by the accelerator pedal
and governor) and then, on to the pumping plungers
where high pressure is developed. This pressurized
fuel compresses the delivery spring, which now allows
fuel movement to a rotor that distributes fuel to the
correct injector. See Figure 29–55. Alignment of the
rotor ports to the pump head determines which cylinder is being supplied with fuel. See Figure 29–56.
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Figure 29–54 Ghost view of a rotary diesel fuel injection
pump. (Courtesy General Motors)
Figure 29–56 Indirect fuel injection injects fuel into a
precombustion chamber. A glow plug is used to ignite the fuel
during cold engine operation. (Courtesy Ford Motor Co.)
Figure 29–55 Fuel flow through a rotary pump fuel
injection system. (Courtesy Ford Motor Co.)
EDFI more closely matches electronic gasoline fuel
injection than the previous mechanical diesel injection systems. One type, the high pressure commonrail electronically controlled diesel injection, was
introduced to Canada and North America in the
later 1990s; pressures in this system may exceed
160 000 kPa (23 000 psi). It is used in both passenger car and light truck applications. See Figure 29–57.
Common-Rail Diesel Fuel
Major components include:
Fuel tanks
Fuel lines
Fuel injector control module
Water separator
Fuel filter
Pump assembly
Fuel rails
Figure 29–57 Schematic of
an electronic diesel fuel
injection system. (Courtesy
General Motors)
Fuel Injector Control Module
Injection Pump
Fuel delivery begins at the pick-up and pre-filter in
the tank; it then flows to the base of the fuel injector
control module (FICM). The module, which requires
93 volts and up to 20 amperes of current to drive the
injectors, is cooled by the fuel flowing through the
base. The FICM is operated by engine control module (ECM) commands.
The engine-driven fuel injection pump generates the
high pressures required for system operation. It includes an ECM-controlled pressure regulator valve,
which varies pump pressure with load: low pressure
at idle, higher pressures with increasing engine load.
Water Sensor-Separator/Primary
Fuel continues to the WSS/PF where it is filtered;
any water in the fuel is separated and collected in
the lower housing. This unit may also contain an integrated hand pump used for priming and a fuel
heater, which is activated in colder temperatures.
Function Block
Fuel moves from the high pressure pump to the function block, which contains both an excess-pressure
limiting valve (acts as a fail-safe relief valve) and a
pressure sensor, which sends fuel pressure readings
to the ECM.
Common Rails
Pressurized fuel arrives at the rails which act as accumulators and reduce fuel pulsing.
Fuel Filters
Fuel filter replacement is a common and essential
service required with diesels as the typical paper element filter becomes restricted.
Electrical Injectors
The injectors are electrical solenoids that function
similarly to electronic gasoline injectors. See Figure
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Figure 29–58 A diesel electrical fuel injector.
Note the fuel return line. (Courtesy General
Figure 29–59. Diesel
engine electronic
management. Note the input
sensors and the output
actuators. (Courtesy
General Motors)
29–58. When activated, the injector coil lifts the needle valve and fuel flows; injector fuel delivery varies
with the on-time (duration) of injector opening.
determined by the on-time that the injector is held
open; increasing the length of time the injector is
opened increases the volume of fuel.
Major information inputs to the ECM (see
Figure 29–59) would include data from the following
The injectors are energized by an electronic control
module (ECM) to begin injection. No power to the injector, no injection. The quantity of fuel delivered is
Mass air flow (MAF)—Measures the intake air
Intake air temperature (IAT) usually located in
the MAF sensor
Accelerator pedal position (APP)—Signals the
driver’s demand for speed and acceleration
Barometric (BARO)—Senses barometric
pressure for fine tuning fuel control
Crankshaft (CKP) and camshaft (CMP) position
sensors—Used to identify engine RPM and
piston location
the fuel injector. The oil, under high pressure, is
blocked by a poppet valve located inside the injector.
When injection is required, an electrical solenoid,
controlled by the Powertrain Control Module (PCM),
opens the poppet valve and oil enters the injector.
This oil acts on the large upper end of a plunger,
which through multiplication of force, injects fuel at
pressures of 18 500 kPa (2700 psi) or higher.
Other input sensors can include fuel pressure, turbocharger (if used) boost, and fuel temperature;
these vary with make and model. The ECM uses this
information to control the fuel injectors and other
various relays. A fuel-injector control module, managed by the ECM, may be used to supply large
amounts of current to drive the injectors.
Diesel engines require normal service and maintenance for different reasons than gasoline engines:
they have no ignition system, no carburetor to
clean, and early diesels have limited emission controls. The following is a list of typical services that
are required:
The HEUI system, used in some light truck applications, is unique in that it uses oil to develop the very
high pressures required to inject diesel fuel. See Figure 29–60.
An engine driven high-pressure oil pump (not
the lubrication pump) delivers oil to the upper end of
Figure 29–60 A high-pressure solenoid controlled fuel
injector. (Courtesy Ford Motor Co.)
Oil and filter change—Because of the high
compression and combustion pressures,
combustion residue (particulates) is blown past
the piston rings and into the oil. Some diesels
use two oil filters to remove contaminants.
Fuel filter replacement—It is essential that
water and foreign material are removed from the
fuel, as they can damage the injection pump and
injectors. Ford is now supplying a long-life fuel
filter on selected models; it is incorporated into
the fuel delivery module and requires no
replacement or service for the life of the vehicle.
Water drainage—Very common service; a
warning light on the instrument panel may also
be used to indicate excessive water in the fuel.
Air filter replacement—Diesels take in far more
air than they normally require because of no
throttle plate; filters are larger than comparable
displacement gasoline engines. An air filter
restriction indicator may be found on the intake
air hose on some models.
Glow plug replacement—Testing of glow plugs
and electrical circuits will be required for coldstart concerns.
Compression testing—For weak piston rings and
valve sealing; see Chapter 5 “Engine Condition
Diagnosis” for details.
Injection pumps and injectors—When a
malfunction is noted with the pump or injectors,
they are usually removed and sent to a diesel
injection specialist for repair or exchange.
The on-board computer (PCM) used with late
model electronic injection, is required to monitor
both engine operation and emission controls;
using a scanner to access data stored in the PCM
memory is also part of normal diesel service with
these models.
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
Storage Oscillocope
Testing a Gasoline Fuel Injector Using a Digital
P20–1 This is the first screen you see when turning on
a Fluke 98 scopemeter.
P20–3 Select “fuel injector” from the air/fuel menu.
P20–5 Use a T-pin to backprobe the injector
connector. These T-pins are usually available at discount
stores and specialty shops in the craft area.
P20–2 Select “air/fuel” from the main menu.
P20–4 The scopemeter will prompt you to connect
the test lead into the input A terminal.
P20–6 Carefully insert the point of the T-pin into the
back of the connector and lightly push on the T-pin
until it contacts the metal terminal inside the
Testing a Gasoline Fuel Injector Using a Digital Storage Oscillocope—continued
P20–7 Attach the test probe from the scopemeter to
the T-pin.
P20–8 Attach the ground test lead to a good, clean
engine ground.
P20–9 Start the engine.
P20–10 Observe the waveform. If the waveform does
not look similar to this, insert the T-pin into the other
terminal of the connector. To achieve this pattern, the
scopemeter should be connected to the terminal that is
being pulsed on and off by the computer. The pulse width
is longer than normal in this photo because the engine is
cold and the computer is pulsing the injector on for a
longer time to provide the engine with additional fuel.
P20–11 Note the shortened pulse width compared to
the previous photo. The engine is now at normal
operating temperature and the injector pulse width
should be 1.5 to 3.5 milliseconds. Also look for
consistent inductive voltage spikes for all injectors,
indicating that the injector coil is not shorted.
P20–12 Turn the engine off and disconnect the
Gasoline and Diesel Fuel Injection: Operation, Diagnosis, and Service
1. A typical throttle-body fuel injector uses a computercontrolled injector solenoid to spray fuel into the
throttle-body unit above the throttle plates.
2. A typical port fuel-injection system uses an individual
fuel injector for each cylinder and squirts fuel directly
into the intake manifold about 75 mm (3 in.) from the
intake valve.
3. Most electric fuel pumps can be tested for pressure,
volume, and current flow.
4. A typical port fuel-injection system fuel pressure should
not drop more than 140 kPa (20 psi) in 20 minutes.
5. A noid light can be used to check for the presence of an
injector pulse.
6. Injectors can be tested for resistance and should be
within 0.3 to 0.4 ohms of each other.
7. Different designs of injectors have different scope
waveform depending on how the computer pulses the
injector on and off.
8. An idle air-control unit controls idle speed and can be
tested for proper operation using a scan tool or scope.
9. Conventional diesel fuel injection controls fuel delivery at the injection pump.
10. Scan tools are used to diagnose electronic diesel injection systems.
1. List the ways fuel injectors can be tested.
2. Describe how to test an electric fuel pump.
3. List the steps necessary to test a fuel pressure
4. Explain why some vehicle manufacturers warn about
using fuel-injector cleaner.
5. Describe why it may be necessary to clean the throttle
plate of a port-injected engine.
6. Describe the operation of a conventional diesel injection system.
7. Explain the operation of an electronic diesel injection
1. How much fuel pressure should most late-model portinjected engines be able to supply?
a. 70 kPa (10 psi)
b. 210 kPa (30 psi)
c. 350 kPa (50 psi)
d. 525 kPa (75 psi)
2. Fuel injectors can be tested using
a. A cylinder balance test
b. An ammeter
c. Visual inspection
d. An ohmmeter
3. Throttle body fuel-injection systems deliver fuel _____.
a. Directly into the cylinder
b. In the intake manifold, near the intake valve
c. Above the throttle plate of the throttle-body
d. Below the throttle plate of the throttle-body
4. Port fuel-injection systems deliver fuel _____.
a. Directly into the cylinder
b. In the intake manifold, near the intake valve
c. Above the throttle plate of the throttle-body
d. Below the throttle plate of the throttle-body
5. The vacuum hose was removed from a vacuummodulated fuel pressure regulator and gasoline
dripped from the hose. This could indicate a
a. Leaking fuel injector
b. Restricted return line
c. Vacuum leak at the regulator hose
d. Leaking pressure regulator diaphragm
6. Fuel pressure drops rapidly when the engine is turned
off. This is normal on some TBI injection systems where
the pressure regulator is equipped with _____.
a. A vacuum line
b. High-pressure fuel injectors
c. A bleed orifice
d. A fuel pressure sensor
7. In a typical port-injection system, the fuel pressure is
regulated _____.
a. By a regulator located on the fuel return side
of the fuel rail
b. By a regulator located on the pressure side of
the fuel rail
c. By the computer by pulsing the regulator on
and off
d. With the one-way check valve in the fuel pump
8. The airflow sensor plate on a K-Jetronic fuel injection
system contacts the control plunger. As the plunger
moves up, the mixture
a. Becomes richer
b. Shuts off because of high RPM
c. Does not change
d. Becomes leaner
9. Returnless fuel injection cycles excess fuel at the pressure regulator in/on the _____.
a. Fuel rail
b. TBI throttle body
c. Fuel tank
d. Fuel rail return line
10. Runaway diesel engines (with conventional injection)
may be stopped by turning off the fuel line shut-off
valve or by
a. Tapping lightly on the injection pump
b. Clamping the low-pressure supply line
c. Opening the water separator drain
d. Stuffing rags into the air intake
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