GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2000 🢒 Bonneville 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Checks

Action

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 Important Preliminary Checks Before Starting in Symptoms .
    • Refer to Visual/Physical Checks in Symptoms .
    • Search for bulletins.

Fuel System
    • Check the fuel pressure. Refer to Fuel System Pressure Test (VIN K) or to Fuel System Pressure Test (VIN 1) .
    • 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/cylinder.
    • Check items which can cause an engine to run rich . Refer to Diagnostic Aids for DTC P0172 Fuel Trim System Rich .
    • Check items that can cause an engine to run lean . Refer to Diagnostic Aids for DTC P0171 Fuel Trim System Lean .

Sensor/System
    • Check for conditions which cause an incorrect idle speed.
       - Check for throttle body tampering, excessive deposits, or damage.
       - Check for a restricted air intake duct or 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 .
    • Check the TP Sensor and related wiring. Refer to DTC P0123 Throttle Position (TP) Sensor Circuit High Voltage .
    • Check the EGR system for proper operation. Refer to DTC P0401 Exhaust Gas Recirculation (EGR) Flow Insufficient .
    • Verify correct operation of the PNP switch by comparing the transaxle range switch input with the vehicle in drive and the gear selector in drive or overdrive.

Ignition System
    • Check for the correct ignition voltage output using the following procedure:
       - Clip the J 26792 spark tester to engine ground.
       - Connect one end of a spark plug wire to the spark tester. Leave the other end of the spark plug wire to the coil being tested.
       - Connect one end of another spark plug wire to the other coil tower. Connect the other end of the spark plug wire to ground.
       - Crank the engine while observing 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 carbon tracking is detected on any component, replace that component and the component's mate.

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

Coil Secondary Resistance
5,000-8,000 ohms

       - Check the spark plug wires for signs of arcing/cross firing, cracks, carbon tracking, plug boot damage, pinched, incorrect routing, or a resistance value greater than the specified value.

Spark Plug Wire Resistance
3280-4921 ohms per meter (1000 ohms-1500 ohms per foot)

   Important: Spraying the secondary ignition wires with a light mist of water may help locate an intermittent problem. Ignition components will arc to ground when a secondary component is malfunctioning.

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

   Important:: If carbon tracking is detected on any component, replace that component and the component's mate.

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

Engine Mechanical
    • Check engine mechanical for the following:
       - Check the compression. Refer to Engine Compression Test in Engine Mechanical.
       - Check for sticking or 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 excessive oil in the combustion chambers or leaking valve seals.
    •  Inspect for the following for incorrect basic engine parts:
       - Inspect the camshaft.
       - Inspect the cylinder heads.
       - Inspect the pistons, etc.
    • Refer to Engine Overhaul in Engine Mechanical for diagnosis procedures.

Additional Checks
    • Check the exhaust system for a restriction.
       - Inspect the exhaust system for damaged or collapsed pipes.
       - Inspect the mufflers for heat distress or internal failure.
       - Check for a plugged 3-way catalytic converter. Refer to Restricted Exhaust System Check .
    • 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 decrease in RPM with little change in the actual engine RPM indicates that EMI is present. Check for incorrect routing of secondary ignition wires. Also check the ignition coils for cracks, carbon tracking, or signs of arcing.
    • Check the intake and exhaust manifold passages for casting flash. Refer to Intake Manifold Cleaning and Inspection in Engine Mechanical.