GM Service Manual Online
For 1990-2009 cars only

Checks

Action

DEFINITION: Steady pulsation or jerking that follows engine speed, usually more pronounced as engine load increases.

Preliminary

    • Refer to Symptoms , Important Preliminary Checks Before Starting.
    • Search for bulletins.
    • Verify that the PCM grounds are clean, tight, and in the correct locations.
    • Inspect the air filter element for dirt and restrictions. Replace the element as necessary.

Fuel System

    •  Perform the fuel injector balance test.
    •  Check for a low fuel pressure. Refer to Fuel System Pressure Test .
    •  Check for a restricted fuel filter.
    •  Check for contaminated fuel. Refer to Fuel Quality Diagnosis .
    • Check items that can cause an engine to run rich.
    • Check items that can cause an engine to run lean.

Sensor and System

    •  Using a scan tool, monitor the knock sensor (KS) system for any excessive spark retard activity.
    • Check the EGR system for the correct operation.

Ignition System

    • Remove the spark plugs and check for the following:
       - Any wet plugs
       - Any cracks
       - Any wear
       - An improper gap
       - Any burned electrodes
       - Any heavy deposits
    • An improper spark plug gap will cause a driveability problem. The spark plug gapping should be done with a wire gauge gap tool. If the spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.
    • Check the plugs and the coils for signs of arcing, cracks, or carbon tracking.
    • Check for any loose ignition control module grounds.
    • Check the spark plugs for the correct heat range.
    • Check for the correct ignition voltage output. Refer to Secondary Ignition Diagnosis .

Engine Mechanical

Check for the following conditions:

    • Any sticking or leaking valves
    • Any worn camshaft lobes
    • An incorrect valve timing
    • Any worn rocker arms
    • Any broken valve springs
    • Any excessive oil in combustion chambers or leaking valve seals
    • A low cylinder compression
    • Any incorrect basic engine parts. Inspect the following:
       - The camshaft
       - The cylinder heads
       - The pistons, etc.

Refer to Base Engine Misfire Diagnosis in Engine Mechanical.

Additional Checks

    • Check the exhaust system for restrictions. Refer to Restricted Exhaust System Check .
    • Check for any improper harness routing. An engine miss condition can be caused by electromagnetic interference (EMI) on some engine controls circuits. Use the scan tool to check for any erratic sensor values or excessive changes in the indicated RPM with little change in the actual engine RPM. This may indicate EMI is present. If a problem exists, check for any improper routing of the harnesses near high voltage components or high current components.
    • Check the charging system. A bad diode in the alternator can allow A/C noise to enter the electrical system. This may affect the engine controls system operation. Refer to Diagnostic Starting Point in Engine Electrical.