GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2002 🢒 Bonneville 🢒 Engine 🢒 Engine Electrical 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Checks

Action

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

Preliminary Checks
    • Refer to Important Preliminary Checks Before Starting in Symptoms - Engine Controls .
    • Refer to visual checks and physical checks in Symptoms - Engine Controls .
    • Search for bulletins.

Fuel System
    • Check the fuel pressure. Refer to Fuel System Diagnosis .
    • Check the fuel injectors. Refer to Fuel Injector Solenoid Coil Test .
    • Check for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Check for fuel in the pressure regulator vacuum hose.
    • Ensure each injector harness is connected to the correct injector and/or to the correct cylinder.
    • Inspect for conditions that can cause a rich fuel mixture. Refer to Diagnostic Aids for DTC P0172 .
    • Inspect for conditions that can cause a lean fuel mixture. Refer to Diagnostic Aids for DTC P0171 .

Sensor/System
    • Check for conditions which can cause an incorrect idle speed.
       - Check for deposits or for damage to the throttle body .
       - Check for a restricted air intake duct or for a dirty air cleaner element. Refer to Air Cleaner Element Replacement .
       - Inspect for a large vacuum leak.
    • Check the crankcase ventilation valve for proper installation. Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical.
    • Check the throttle position (TP) sensor and the related wiring. Refer to DTC P0123 .
    • Check the exhaust gas recirculation (EGR) system for proper operation. Refer to DTC P0401 .
    • Verify the correct operation of the park neutral position (PNP) switch. Compare the transaxle range switch input with the vehicle in drive and with the gear selector in drive or in overdrive.

Ignition System
    • Check for the correct ignition voltage output using the following procedure:
       - Attach the J 26792 Spark Tester to engine ground.
       - Connect one end of a spark plug wire to the spark tester. Connect the other end of the spark plug wire to the coil that is being tested.
       - Connect another spark plug wire between the remaining tower of the coil that is being tested and the ground.
       - Crank the engine, and observe the spark tester. A spark should be observed.
       - Repeat the above steps for each coil.
    • If spark is not present at the coils, check for the following conditions:

   Important:: If you detect carbon tracking on any component, replace that component and the component's respective match.

       - Check the coils for cracks, for carbon tracking and/or arcing, or for a resistance value outside the specified range.

Secondary Coil Resistance
5,000-8,000 ohms

       - Check the spark plug wires for the following conditions: cracks, any arcing, incorrect firing, incorrect routing, carbon tracking, plug boot damage, pinched wires, or a resistance value greater than the specified value.

Spark Plug Wire Resistance
3280-4921 ohms per meter (1,000--1,500 ohms per foot)

   Important: Spraying the secondary ignition wires with a small amount of water may help locate an intermittent condition. Ignition components will arc to ground if a secondary component is malfunctioning.

       - Check the ignition control module (ICM) for a malfunction.
       - Check the ignition system wiring for a loose ignition module feed, a bad ground connection, or for a damaged wiring system .
    • Remove the spark plugs.
    • Inspect the spark plugs for the following conditions:
       - Fouling
       - Cracks

   Important:: If you detect carbon tracking on any component, replace that component and the component's respective match.

       - Check the spark plugs for signs of carbon tracking on the porcelain or for any arcing at the plug wire contact.
       - Check the spark plugs for an incorrect gap. Refer to Maintenance Items in Maintenance and Lubrication.
       - Check the spark plugs for damaged electrodes.
       - Check the spark plugs for an incorrect heat range. Refer to Maintenance Items in Maintenance and Lubrication.
    • If spark plugs are fouled from gas or from oil, the cause of the fouling must be determined before replacing the spark plugs.

Engine Mechanical
    • Check in Engine Mechanical for the following conditions:
       - Check the compression. Refer to Engine Compression Test in Engine Mechanical.
       - Check for sticking or for leaking valves. Refer to Cylinder Leakage Test in Engine Mechanical.
       - Inspect for worn camshaft lobes.
       - Check the valve timing.
       - Check for bent push rods.
       - Check for worn rocker arms.
       - Check for broken valve springs.
       - Check for excess oil in the combustion chambers and for leaking valve seals.
    • Inspect for incorrect engine parts, such as the following basic components:
       - The camshaft
       - The cylinder heads
       - The pistons
    • Refer to Symptoms - Engine Mechanical in Engine Mechanical.

Additional Checks
    • Check the exhaust system for a restriction.
       - Inspect the exhaust system for damaged pipes or for collapsed pipes.
       - Inspect the mufflers for heat damage or for an internal condition.
       - Check for an obstructed three-way catalytic converter. Refer to Restricted Exhaust .
    • Check for loose, worn, or broken motor mounts. Refer to Engine Mount Inspection in Engine Mechanical.
    • 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 or a sudden decrease in RPM with little change in the actual engine RPM indicates that EMI is present. Check for incorrect routing of secondary ignition wires. Check the ignition coils for cracks, for carbon tracking, or for signs of arcing.
    • Check the intake and the exhaust manifold passages for casting flash. Refer to Upper Intake Manifold Replacement and Lower Intake Manifold Replacement in Engine Mechanical.