Wrangler JK Engine Cooling
August 30, 2017
Wrangler JK Engine Cooling
In the old days the temperature gauge was a simple and direct monitor of engine
cooling. Contemporary cars with their recovery tanks, electric fans, and electronic
controls provide more efficient and optimal control of engine operating temperature.
But the added components also provide more failure modes. Bad things can happen
before they show up on the temperature gauge or in a Diagnostic Trouble Code.
Here I describe what to expect from a normally operating system. I also describe a
problem that didn't show on the gauge and didn't “throw a code”.
The particular vehicle in this case is a 2009 Jeep Wrangler JK with the 3.8l engine.
The principals should also apply to most vehicles.
Copyright 2017, Steve Harlow
Wrangler JK Engine Cooling
August 30, 2017
What to Expect During Normal Operation
Here is what you should see with everything working normally. Begin with the
engine cold. (Typically 70 F ambient in my case.)
Squeeze the upper radiator hose. It shouldn't be hard to squeeze it to half its rest
diameter. It should not collapse all the way. There should be no bubbling or gurgling
sound. (I hear a little clicking, probably coming from a part of the thermostat.)
If the hose collapses or you hear bubbles then you have air in the system. This may be
a minor problem or it could be a symptom of something more serious. (Possible
causes include empty recovery tank, bad radiator cap, or even blown head gasket.)
If there is air it needs to be purged. A normally operating system can purge itself of a
small amount (half a liter?) of air. If there is too much air it can't recover and bad
things will happen (or have already happened). I won't cover purging here. You can
find references on the internet.
Copyright 2017, Steve Harlow
Wrangler JK Engine Cooling
August 30, 2017
Check the level in the recovery tank. It should be between the min and the max
marks. Take note of your level.
I drew some extra marks on my tank so as to better see what is happening.
Start the engine.
After about 3 minutes the dash board temperature gauge should start to show
something above “cold”.
After about 10 minutes the gauge needle will be about one small division below mid
scale. That corresponds to a coolant temperature of about 170 F. The thermostat will
still be closed and the upper radiator hose will not be hot. You should be able to get
good heat from the heater.
After a couple more minutes the temperature should be about 195 F. The thermostat
will open and the upper radiator hose will get hot. The gauge needle will be right at
mid scale (or a hair below).
Copyright 2017, Steve Harlow
Wrangler JK Engine Cooling
August 30, 2017
The level in the recovery tank should also have risen by between 1 and 1.5 inches. It
should not rise more than a couple tenths of an inch hereafter.
After another 5 minutes the electric radiator fan should turn on at its low speed. The
temperature gauge will still show at mid scale as above. The recovery tank level
might be very slightly higher as just mentioned.
After about a minute the fan will turn off. Then after another 3 minutes the fan will
turn on again. It will continue to cycle that way, 1 minute on, 3 minutes off. There
will be practically no change in the temperature gauge indication.
When you turn the engine off and it cools off, the recovery tank level will eventually
return to approximately its starting level. That will take hours.
Copyright 2017, Steve Harlow
Wrangler JK Engine Cooling
August 30, 2017
The Insidious Problem
I had a problem with my vehicle that wasn't real obvious at first.
 recovery tank level at Min mark when cold and at Max mark when hot.
 Occasional bubbling, gurgling sound coming from engine when pausing after
slow driving.
 Radiator fan turning on occasionally
 Temperature gauge never showed above normal (mid scale).
 No Diagnostic Trouble Codes
 Some engine knock heard while under load at low speed
 Coolant system full. No air in the system when cold.
If you are a fan of Car Talk and like to see if you can figure out the problem before
Click and Clack then pause here before continuing.
Copyright 2017, Steve Harlow
Wrangler JK Engine Cooling
August 30, 2017
Trouble Shooting
First I calculated how much the recovery tank level should actually rise. I looked up
the expansion coefficient of the coolant, coolant capacity, and thermostat setting. I
calculated the level should rise a little more than an inch. So when it rose from the
Min mark to the Max mark that was definitely a sign of a problem.
The first easy possible fix came to mind, radiator cap. Maybe it wasn't holding
pressure and coolant was boiling at lower temperature. It would be a cheap and easy
fix. $5. Tried it. No joy.
But in testing it I noticed that if I started and immediately got rolling the recovery tank
level rose the 1+ inch and stayed there. Then when I stopped and let it idle, after a
few minutes the level would rise to the Max mark in a couple minutes time. Then the
fan would come on. After shutting the engine off and waiting 10 minutes the level
started falling quickly. In a couple minutes it was back down to the 1+ inch level. So
obviously the big rise was steam expansion and the quick fall was the steam
condensing again. The temperature gauge was showing at the center the whole time.
I decided to do a controlled test. Cold start, idle in the garage, and measure
everything. I had an IR thermometer to measure the temperature of the manifold by
the engine coolant temperature (ECT) sensor. I used an OBDII interface (BAFX
interface and Torque Pro phone app) to read the ECT sensor value.
I'll spare you all of the details but there were two significant observations. When the
boiling sound was heard the OBDII was showing 212 F. That temperature is
significantly lower than that at which coolant should boil at in a pressurized system.
And the IR thermometer was showing about 237 F on the outside by the ECT sensor.
These things suggested that the ECT sensor was defective, not a hard failure but
giving a low reading.
I replaced the ECT sensor. And that is what the problem was. What are the odds of
that? The soft failure generally explains why it didn't throw a code. Maybe it would
have eventually failed hard. In the mean time it was stressing the engine. It was
running hotter, allowing steam to displace coolant, and causing engine knock.
Copyright 2017, Steve Harlow
Wrangler JK Engine Cooling
August 30, 2017
The ECT sensor costs about $26 at most auto parts stores. It comes with some sealing
material already on the threads.
I was going to drain some coolant so it wouldn't run out when I removed the sensor.
But the radiator drain is inconvenient to access. (Cheesy plastic fasteners hold the
damn air dam.) So I thought I'd put rags around the sensor and do a quick swap. That
wasn't necessary. The coolant sat there and didn't run out.
If you don't do this kind of thing every day then disconnecting the connector might be
a minor puzzle. First you pull the red part up about a quarter inch. Then you push in
on the black tab while wiggling the connector off.
Copyright 2017, Steve Harlow
Wrangler JK Engine Cooling
August 30, 2017
Sensor Post Mortem
I measured the performance of the new and old sensors for comparison and
verification. The sensor is a negative temperature coefficient thermistor. It's
resistance goes down with increasing temperature. I measured the performance with
an ohmmeter, a pot of water on the stove, and a cooking thermometer. Below is a
graph of the results.
So, yeah, the bad one was bad. About 25 to 30 F off. Who would have expected such
a sneaky failure mode.
Copyright 2017, Steve Harlow
Wrangler JK Engine Cooling
August 30, 2017
Interesting Facts
I did a test run from a cold start and recorded engine coolant temperature (with the
BAFX and Torque Pro). Samples were taken once per second. The results are plotted
Before the engine reaches normal operating temperature (about 195 F) the thermostat
is closed.
In the graph you see a spike in temperature at the 300 second point. I don't have a
definite explanation for that. Maybe the thermostat sticks a bit when it first opens.
Copyright 2017, Steve Harlow
Wrangler JK Engine Cooling
August 30, 2017
When the engine is warmed up and vehicle motion is giving good air flow over the
radiator then the thermostat regulates ECT. The thermostat is set at 195 F. From
other observations it seems that the vehicle has to be going about 35 mph or better (at
80 F ambient) to get enough air flow to allow the thermostat to do the regulation.
When going slow or standing still the Powertrain Control Module (PCM) basically
takes over ECT regulation (using the electric fan). When the PCM sees ECT reach
213.8 F it turns the fan on (at its low speed). When the ECT gets back down to 203.8
the PCM turns the fan off. The thermostat will be fully open during this time.
Note that the coolant level in the recovery tank acts like a thermometer. The level
gives a general idea of the ECT. The coolant expands with temperature, just like the
fluid in a thermometer. (The engine expands too but not as much.) So if the engine is
cooled to an ambient of 80 F and you run it up to operating temperature, 195 F, the
level will rise about an inch (with my '09 JK). If you drive it to somewhere where the
ambient is 30 F overnight then the next morning you'll find the level about a half inch
lower than it was at the 80 F ambient.
Copyright 2017, Steve Harlow
Wrangler JK Engine Cooling
August 30, 2017
The temperature gauge is not linear. Below is a picture of the gauge with actual
temperature values marked.
On the left side the small divisions are about 10 F each. Then there are about 50
degrees crammed into the one division on the left side of center scale. This is
apparently by design. I suppose they didn't want the customer to see the 20 F swings
when the fan runs.
The gauge is controlled by the PCM. The PCM reads the ECT sensor and then sends
a serial digital message to the instrument cluster via a CAN bus. The message tells
the gauge what angle to set the needle. Obviously it is controlled by software. The
software apparently scales/compresses the gauge indication.
Copyright 2017, Steve Harlow
Wrangler JK Engine Cooling
August 30, 2017
System Basics
Most of you probably already know this stuff. Here is a system level overview.
Combustion is affected by cylinder and head temperature. The cooling system tries to
control the temperature at a relatively fixed optimum temperature. Then the spark
timing and mixture are adjusted for optimum combustion depending on load, air
density, etc.
When the engine is cold the thermostat is closed. That blocks flow through the
radiator. The water pump recirculates the coolant around the block and heads. The
coolant bypasses the radiator by going through the bypass. (Note that in the JK the
bypass is actually through the cabin heater core.) So overall, cooling is reduced to
quickly get the engine up to optimal operating temperature.
When the coolant gets up to the design temperature the thermostat starts to opens.
That allows coolant to flow through the radiator. The thermostat opens wider at
Copyright 2017, Steve Harlow
Wrangler JK Engine Cooling
August 30, 2017
higher temperature. That has the effect of regulating the coolant temperature in a
narrow range. That is, as long as there is good air flow through the radiator.
When the vehicle is moving slow or stopped there isn't enough air flow through the
radiator. The ECT rises. The thermostat is fully open but that can't keep the
temperature in range. The PCM sees the ECT rising and turns on the fan. It controls
the fan to keep the temperature within limits.
The electric fan system allows the vehicle to have a smaller radiator. The radiator
doesn't have to be big enough to radiate all the heat when the vehicle is stopped and
the engine is idle. And it is more efficient in that no energy is put into turning the fan
when traveling at highway speed and there is already sufficient air through the
Side note: ambient air is typically 100 F cooler than the target engine coolant
Copyright 2017, Steve Harlow