GM Service Manual Online
For 1990-2009 cars only

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DEFINITION: Engine runs unevenly at idle. If severe, the engine or vehicle may shake. Engine idle speed may vary in RPM. Either condition may be severe enough to stall the engine.

Preliminary Checks

    • Refer to Symptoms - Engine Controls .
    • Search for service bulletins.

Fuel System

    • Inspect system fuel pressure. Refer to Fuel System Diagnosis .
    • Inspect the fuel injectors. Refer to Fuel Injector Balance Test with Tech 2 .
    • Inspect the fuel composition sensor for accuracy. Perform the Fuel Composition Test and compare the results.
    • Inspect the fuel filter for being plugged or restricted.
    • Inspect items which can cause an engine to run rich-- Short term FT significantly in the negative range. Refer to Diagnostic Aids for DTC P0172 .
    • Inspect items that can cause an engine to run lean-- Short term FT significantly in the positive range. Refer to Diagnostic Aids for DTC P0171 .
    • Inspect for proper operation of the evaporative emission (EVAP) control system.

Sensor/System

    • If the stalling condition occurs when the A/C is turned ON or when the transmission is put into gear, make sure the powertrain control module (PCM) recognizes the change in state for the component and the idle air control (IAC) counts change accordingly
    • Inspect for proper IAC operation using the scan tool.
    • Inspect the oxygen sensor (O2S 1). The O2S 1 should respond quickly to different throttle positions. If not, check 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.
    • Inspect for proper operation of the manifold absolute pressure (MAP) sensor.
    • Inspect for proper operation of the throttle position (TP) sensor.
    • Inspect for proper operation of the engine coolant temperature (ECT) sensor.

Ignition System

    • Inspect for proper ignition voltage output with a J 26792 Spark Tester or equivalent.
    • Check the spark plugs for the following conditions:
       - Wet or fuel fouled spark plugs
       - Cracks on the spark plugs
       - Wear
       - Improper spark plug gap
       - Burned 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.
    • Inspect the ignition coils for cracks or carbon tracking.
    • Inspect the spark plug wires for the proper resistance and condition.

Engine Mechanical

    • Inspect engine mechanical for the following:
       - Compression--Refer to Engine Compression Test in Engine Mechanical 2.2L (L43).
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Bent push rods
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in combustion chamber and leaking valve seals
    • Inspect for the following incorrect basic engine parts:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.
    • Refer to Symptoms - Engine Mechanical in Engine Mechanical 2.2L (L43).

Additional Checks

Inspect the following components of the exhaust system for possible restrictions:

    • Inspect exhaust system for damaged or collapsed pipes.
    • Inspect mufflers for heat distress or possible internal failure.
    • For possible plugged TWC--Refer to Restricted Exhaust in Engine Exhaust.
    • An engine miss condition can be caused by electromagnetic interference (EMI) on the CKP reference circuit. EMI can usually be detected by monitoring medium resolution counter with a scan tool. A reading other than zero indicates EMI is present. If a problem exists, check routing of secondary ignition wires, high voltage components--Near ignition control circuits.
    • Inspect for malfunctioning engine mounts.