GM Service Manual Online
For 1990-2009 cars only

Checks

Action

DEFINITION: A surging or jerking that follows engine speed, usually more pronounced as the engine load increases.

Preliminary Checks

Refer to Important Preliminary Checks in Symptoms - Engine Controls .

Fuel System Checks

    • Test for proper fuel system pressures. Refer to Fuel System Diagnosis .
    • Inspect for proper operation of the cng fuel injectors. Refer to:
    • Test the high and low pressure lock-off valves / circuits for proper operation.

Ignition System Checks

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

    • Check for proper secondary ignition system voltage output using ONLY the J 26792 Spark Tester.
    • Inspect for fouled spark plugs.
        If fouled spark plugs are indicated the reason for the fouling must be found and corrected.
    • Test for faulty spark plug wires.
    • Test for an open ignition system ground, if equipped.

Engine Mechanical Checks

Test for proper engine operation by checking for the following:

    • Correct engine compression
    • Correct engine valve timing
    • A worn or incorrect camshaft
    • Bent pushrods
    • Worn rocker arms
    • Casting flash in the intake or exhaust manifold passages

Additional Checks

Inspect for electromagnetic interference (EMI) by checking for the following:

    • Monitoring engine speed on the scan tool may indicate EMI is present.
        A sudden increase in scan tool engine speed while actual engine speed remains constant indicates EMI is present.
    • EMI on the ignition control circuit may cause a missing condition.