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 - Engine Controls , Important Preliminary Checks Before Starting.
    • Search for bulletins.
    • Verify that the powertrain control module (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.
    •  Inspect for a low fuel pressure. Refer to Cuts Out, Misses .
    •  Inspect for a restricted fuel filter.
    •  Inspect for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect items that can cause an engine to run rich.
    • Inspect 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.
    • Inspect the exhaust gas recirculation (EGR) system for the correct operation.

Ignition System

    • Remove the spark plugs. Inspect 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 gage gap tool. If the spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.
    • Inspect the plugs and the coils for signs of arcing, cracks, or carbon tracking.
    • Inspect for any loose ignition control module grounds.
    • Inspect the spark plugs for the correct heat range.
    • Inspect for the correct ignition voltage output. Refer to Electronic Ignition (EI) System Diagnosis .

Engine Mechanical

Inspect 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 Symptoms - Engine Mechanical in Engine Mechanical-3.5L.

Additional Checks

    • Inspect the exhaust system for restrictions. Refer to Restricted Exhaust in Engine Exhaust.
    • Inspect 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 inspect 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, inspect for any improper routing of the harnesses near high voltage components or high current components.
    • Inspect 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 - Engine Electrical in Engine Electrical.