GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Oldsmobile 🢒 1999 🢒 Eighty Eight 🢒 Engine 🢒 Engine Controls - 3.8L 🢒 Rough, Unstable, or Incorrect Idle and Stalling

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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 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) .
       - Refer to Fuel System Pressure Test (VIN 1) .
    • Check the fuel injectors.
    • 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 (long term fuel trim is significantly in the negative range). Refer to Diagnostic Aids for DTC P0172 Fuel Trim System Rich .
    • Check items that can cause an engine to run lean (long term fuel trim is significantly in the positive range). 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 .
    • Check the Transaxle Range Switch input with the vehicle in drive and the gear selector in drive or overdrive. Refer to the Transaxle Range Switch Valid Combinations table in DTC P0705 Trans Range Switch Circuit .

Ignition System
    • Check for proper ignition voltage output using the following steps:
       - Clip the spark tester J 26792 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 crisp blue spark should be observed.
       - Repeat the above steps for each coil.
    • If adequate spark is not present at the coil(s), check for the following conditions:

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

       - Check the coil(s) for cracks, carbon tracking/arcing, or a resistance value outside the specified range.

Coil Secondary Resistance
5000 - 8000 ohms (5K - 8K 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.

VIN 1 Spark Plug Wires Resistance
3280 - 4921 ohms per meter (1000 ohms - 1500 ohms per foot)

VIN K Spark Plug Wires 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:
       - Check the spark plug(s) for fouling. Refer to Spark Plug Visual Diagnosis .
       - Check the spark plug for cracks. Refer to Spark Plug Visual Diagnosis .

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

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

Engine Mechanical
    • Check engine mechanical for the following:
       - Check the compression.
       - Check for sticking or leaking valves.
       - Inspect for worn camshaft lobe(s).
       - Check the valve timing.
       - Check for bent push rods.
       - Check for worn rocker arms.
       - Check for broken valve springs.
       - Check for excessive oil in combustion chamber - Leaking valve seals.
    • For incorrect basic engine parts. Inspect for the following. Refer to Engine Overhaul .
       - Inspect the camshaft.
       - Inspect the cylinder heads.
       - Inspect the pistons, etc.
    • Refer to Engine Overhaul for diagnosis procedures.

Additional Checks
    • Check the exhaust system for a possible restriction.
       - Inspect the exhaust system for damaged or collapsed pipes.
       - Inspect the mufflers for heat distress or possible internal failure.
       - Check for possible plugged Three-Way catalytic converter. Refer to Restricted Exhaust System Check and/or Restricted Exhaust .
    • Check for a loose, worn, or broken motor mount(s). Refer to Engine Mount Inspection .
    • 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 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 for loose, worn or broken motor mounts. Refer to Engine Mount Inspection .
    • Check for intake and exhaust manifold passages for casting flash. Refer to Intake Manifold Cleaning and Inspection .