GM Service Manual Online
For 1990-2009 cars only

Checks

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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 , 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 for restrictions. Replace the element as necessary.

Fuel System

    • Perform the fuel injector balance test. Refer to Fuel Injector Balance Test with Tech 2 .
    • Check the fuel pressure. Refer to Cuts Out, Misses .
    • Check for a restricted fuel filter.
    • Check for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Check for an incorrect minimum idle speed.
    • Verify that each fuel injector harness is connected to the correct injector and cylinder according to the firing order. Relocate any fuel injector harnesses as necessary.
    • Check for fuel injectors that are shorted internally. Compare the injector resistances.
    • Check for fuel in the pressure regulator vacuum hose. If any fuel is present, replace the regulator assembly.
    • Check for any foreign material in the throttle bore, and for coking on the throttle valve or on the throttle shaft. Also inspect for any throttle body tampering.
    • Check the items that can cause an engine to run rich.
    • Check the items that can cause an engine to run lean.

Sensor and System

    • Check the crankcase ventilation valve for the correct operation. Place a finger over the inlet hole in the valve end several times. The valve should snap back. If not, replace the valve.
    • Using a scan tool, monitor the throllte position (TP) sensor voltage. The TP sensor voltage should be less than 0.9 volt at rest. If the TP sensor voltage is more than 0.9 volt, inspect the throttle body and the accelerator cables for binding.
    •  Using a scan tool, monitor the knock sensor (KS) system for any excessive spark retard activity.
    • Check the exhaust gas recirculation (EGR) system for the correct operation.

Ignition System

Remove the spark plugs and check for the following conditions:

    • Any wet plugs
    •  Any cracks
    •  Any wear
    •  Any burned electrodes
    •  Any heavy deposits
    •  An improper gap
        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 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 Electronic Ignition (EI) System Diagnosis .

Engine Mechanical

Check for the following conditions:

    • Any excessive oil in the combustion chambers may indicate leaking valve seals. Refer to Oil Consumption Diagnosis in Engine Mechanical.
    • A low cylinder compression--Refer to Engine Compression Test in Engine Mechanical.
    • Any incorrect basic engine parts--Inspect the following parts:
       - The camshaft
       - The cylinder heads
       - The pistons, etc.

Refer to Symptoms - Engine Mechanical in Engine Mechanical.

Additional Checks

    • Visually and physically check the vacuum hoses for splits, kinks, and the correct connections and routing as shown on the Vehicle Emission Control Information label.
    • Check the torque converter clutch (TCC) operation. A scan tool should indicate an RPM drop when the TCC is commanded ON. Refer to Diagnostic Starting Point - Automatic Transmission in Automatic Transaxle.
    • Check the exhaust system for a restriction. Refer to Restricted Exhaust in Engine Exhaust.
    •  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 an improper routing of the harness near the high voltage components or the 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 - Engine Electrical in Engine Electrical.
    • Check the transaxle range switch circuit. Ensure the scan tool indicates the vehicle is in drive with the gear selector in Drive or in Overdrive.