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 .
    •  Search for service bulletins.

Fuel System

    • Test for proper fuel pressure. Refer to Fuel System Diagnosis .
    • Test the fuel injectors. Refer to Sequential Multiport Fuel Injector Balance Test .
    • Test for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Test the fuel filter for being plugged or restricted.
    • Inspect items which can cause an engine to run rich (long term fuel trim significantly in the negative range). Refer to Diagnostic Aids for DTC P0172 Fuel Trim System Rich .
    • Inspect items that can cause an engine to run lean (Long Term FT significantly in the positive range). Refer to Diagnostic Aids for DTC P0171 Fuel Trim System Lean .
    • Test for proper operation of the evaporative emission (EVAP) control system.
    • Inspect for fuel in the fuel pressure regulator vacuum hose.

Sensor/System

    • The O2S 1 should respond quickly to different throttle positions. If not, inspect the O2S 1 for silicon or other contaminants from fuel or the use of improper RTV sealant. The sensors may have a white powdery coating. Silicon contamination causes a high but false O2S 1 signal voltage (rich exhaust indication). The PCM will reduce the amount of fuel delivered to the engine, causing a severe driveability problem. Refer to oxygen sensor in Information Sensors/Switches Description .
    • Test for proper operation of the following sensors:
       - MAP
       - TP
       - ECT

Ignition System

    • Test for proper ignition voltage output with a J 26792 Spark Tester or equivalent.
    • Inspect the spark plugs for the following conditions:
       - Wet or fuel fouled
       - Cracks
       - Wear
       - Improper gap
       - Burnt electrodes or heavy deposits
    • If the spark plugs are fuel or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.
    • Inspect the ignition control module (ICM) for proper ground connection.
    • Test the ignition coils for cracks or carbon tracking.
    • Test/inspect the spark plug wires for proper resistance and condition.

Engine Mechanical

Inspect the engine for the following engine mechanical problems:

    • Low compression, refer to Engine Compression Test in Engine Mechanical
    • Correct valve timing, refer to Camshaft Timing Chain and Sprocket Cleaning and Inspection in Engine Mechanical
    • Leaking cylinder head gasket, refer to Engine Compression Test in Engine Mechanical
    • Worn or a incorrect camshaft, refer to Camshaft and Bearings Cleaning and Inspection in Engine Mechanical
    • Check engine mechanical for the following conditions:
       - Compression, refer to Engine Compression Test in Engine Mechanical
       - Sticking or leaking valves, refer to Valve Train Diagnosis in Engine Mechanical
       - Worn camshaft lobes, refer to Camshaft and Bearings Cleaning and Inspection in Engine Mechanical
       - Valve timing, refer to Camshaft Timing Chain and Sprocket Cleaning and Inspection in Engine Mechanical
       - Bent push rods, refer to Valve Train Diagnosis in Engine Mechanical
       - Worn rocker arms, refer to Valve Train Diagnosis in Engine Mechanical
       - Broken valve springs, refer to Valve Train Diagnosis in Engine Mechanical
       - Excessive oil in combustion chamber -- Leaking valve seals, refer to Oil Consumption Diagnosis in Engine Mechanical
    • For incorrect basic engine parts. Inspect the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Additional Checks

    • Inspect the exhaust system for the following conditions:
       - Damaged or collapsed pipes
       - Mufflers for heat distress or possible internal failure
       - Plugged Three-Way catalytic converter, refer to Restricted Exhaust System Check .
    •  An engine miss condition can be caused by Electromagnetic Interference (EMI) on the reference circuit. EMI can usually be detected by monitoring engine RPM with a scan tool. A sudden increase in RPM with little change in actual engine RPM change, indicates EMI is present. If a problem exists, inspect routing of secondary ignition wires, high voltage components (near ignition control circuits).
    • Inspect for intake and exhaust manifold passages for casting flash. Refer to Intake Manifold Cleaning and Inspection and Exhaust Manifold Cleaning and Inspection in Engine Mechanical.
    • Inspect for malfunctioning motor mounts. Refer to Engine Mount Inspection in Engine Mechanical.