PG-08 Diagnostics and Trouble Shooting Guide

Manual 2012 Diag Diagnostic Trouble shooting GM Engines

Basic Troubleshooting (cont’d.)

Surges and/or Stumbles

Engine power varies under steady throttle or cruise. Feels like the vehicle speeds up and slows down with no change in the acceleration pedal.

PRELIMINARY CHECKS

Perform the visual checks as described at start of “ Basic Troubleshooting” chapter.

Be sure driver understands vehicle operation as explained in the operator manual.

PROBABLE CAUSE CORRECTIVE ACTION

Oxygen sensor malfunction

Fuel system malfunction

The fuel management should maintain a stoichiometric air-fuel ratio under all steady state operating conditions following engine warmup.

Failure of the Pre-catalyst O2 sensor should cause an O2 sensor fault that can be diagnosed with the

MIL lamp or Service Tool.

NOTE: To determine if the condition is caused by a rich or lean system, the vehicle should be driven at the speed of the complaint. Monitoring precatalyst O2 adapts* or dither valve duty cycle will help identify problem.

Check fuel supply while condition exists.

Check in-line fuel filter. Replace if dirty or plugged.

Check fuel pressure.

Ignition system malfunction

Component malfunction

Check for proper ignition voltage output using spark tester.

Check spark plugs.

•

Remove spark plugs, check for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits.

•

Repair or replace as necessary.

•

Check condition of distributor cap, rotor and spark plug wires (where applicable).

•

Check ignition timing.

Check vacuum lines for kinks or leaks.

Check alternator output voltage. Repair if less than

9 or more than 16 volts.

Exhaust backpressure

Check condition of exhaust system.

Check backpressure before catalyst. It should be less than 3.5 psig (24.13 kPa).

(

*

) Refer to Table 1 for description of gaseous O2 adapts.

Related MIL Faults:

Pre-catalyst O2 sensor errors / O2 control errors

Dither valve DC faults / EST faults / ETC faults

Buck’s Engines 6