GM Service Manual Online
For 1990-2009 cars only

Checks

Action

DEFINITION: The fuel ignites in the intake manifold, or in the exhaust system, making a loud popping noise.

Preliminary Checks

Refer to Important Preliminary Checks in Symptoms - Engine Controls .

Ignition System Checks

       Important:: A higher secondary ignition voltage is required to operate the vehicle on CNG than on gasoline.

    • Inspect for proper secondary ignition system voltage output using only the J 26792 spark tester.
    • Remove and inspect the spark plugs for the following conditions:
       - Wet plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned electrodes
       - Heavy deposits
    • Test for proper ignition system operation.
    • Test for crossfire between spark plugs. Test for spark plug wire failure. Wetting down the secondary ignition system with a water spray bottle may aid in locating damaged or deteriorated components.

Engine Mechanical Checks

    • Inspect the engine for the following:
       - Intake manifold leaks
       - Improper valve timing
       - Proper engine cylinder compression
       - Sticking or leaking intake or exhaust valves
       - Exhaust system leakage
    • Inspect the intake and exhaust system for casting flash.

Fuel System Checks

Perform fuel system diagnosis. Refer to Fuel System Diagnosis .

Additional Checks

    • Review service bulletins for AF ECM or PCM programming updates.
    • Verify that the correct CNG software and calibration has been programmed into the PCM.
    • Test for proper operation of the AFO Enable circuit.