GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Oldsmobile 🢒 1999 🢒 Intrigue 🢒 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.
    • Refer to Visual/Physical Checks
    • Search for bulletins.

Fuel System
    • Test the fuel pressure. Refer to Fuel System Pressure Test .
    • Test for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect for fuel in the pressure regulator vacuum hose.
    • Test the items which can cause an engine to run rich. Refer to DTC P0172 Fuel Trim System Rich .
    • Test the items that can cause an engine to run lean. Refer to DTC P0171 Fuel Trim System Lean .

Sensor/System
    • Inspect for conditions which cause an incorrect idle speed.
       - Any throttle body tampering, excessive deposits, or damage. Refer to Fuel Metering Modes of Operation .
       - A restricted air intake system. Refer to Air Intake System Description .
       - A large vacuum leak.
    • Test the crankcase ventilation valve for proper operation. Refer to Crankcase Ventilation System Description .
    • Test the throttle position (TP) Sensor and the related wiring. Refer to DTC P0123 Throttle Position (TP) Sensor Circuit High Voltage .
    • Test the EGR system for proper operation. Refer to Exhaust Gas Recirculation (EGR) System Description .
    • Test the transaxle range switch input with the vehicle in drive and the gear selector in drive or overdrive.

Ignition System
    • Test for proper 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 connected 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. Spark should be observed.
       - Repeat the above steps for each coil.
    • If adequate spark is not present at the coils, inspect for the following conditions:
       - Inspect the coils for cracks, carbon tracking/arcing, or a resistance value outside the 5000 - 8000 ohms range.
       - Inspect the spark plug wires for signs of arcing/cross firing, cracks, carbon tracking, plug boot damage, pinched wires, improper routing, or a resistance value other than 600 ohms per ft. (1968 ohms per meter).

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

       - Test for a defective module.
       - Inspect the ignition system wiring for a loose ignition module feed or ground connection, or damaged system wiring.
    • Remove the spark plugs and inspect for the following:
       - Fouled plugs
       - Cracks
       - Wear
       - Improper gap
       - Burned or damaged electrodes
       - Improper heat range or reach
    • If spark plugs are gas or oil fouled, the cause of the fouling must be determined before replacing the spark plugs.

Engine Mechanical
    • Test or inspect the following engine mechanical components for the following:
       - Test the compression.
       - Test for sticking or leaking valves.
       - Inspect for worn camshaft lobe(s).
       - Test the valve timing.
       - Inspect for bent push rods.
       - Inspect for worn rocker arms.
       - Inspect for broken valve springs.
       - Test for excessive oil in combustion chambers or leaking valve seals.
    • Inspect for incorrect basic engine parts. Inspect the following:
       - The camshaft
       - The cylinder heads
       - The pistons, etc.

Additional Checks
    • Inspect the exhaust system for possible restrictions. Inspect for the following:
       - Inspect the exhaust system for damaged or collapsed pipes.
       - Inspect the mufflers for heat distress or possible internal failure.
       - Test for a plugged 3-way catalytic converter. Refer to Restricted Exhaust System Check and Exhaust System Inspection in Engine Exhaust.
    • An engine miss condition can be caused by electromagnetic interference (EMI) on the reference circuit. EMI can usually be detected by monitoring the engine RPM with a scan tool. A sudden increase in RPM with little change in the actual engine RPM, indicates that EMI is present. If a problem exists, inspect the routing of the secondary ignition wires near high voltage components.
    • Inspect for faulty motor mounts. Refer to Engine Mount Inspection .
    • Check for intake and exhaust manifold passages for casting flash. Refer to Intake Manifold Cleaning and Inspection Exhaust Manifold Cleaning and Inspection in Engine Mechanical.