GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Oldsmobile 🢒 1998 🢒 LSS 🢒 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
    • Check the fuel pressure.
       - Refer to Fuel System Pressure Test (VIN K) .
       - Refer to Fuel System Pressure Test (VIN 1) .
    • Check the 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.
       - Throttle body tampering, excessive deposits, or damage.
       - Restricted air intake duct or dirty air cleaner element. Refer to Air Cleaner Element Replacement .
       - 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 coils, check for the following conditions:
       - Coils - Cracks, carbon tracking/arcing, or a resistance value outside the specified range.

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

       - Spark Plug wires - 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 - 1500 ohms per foot).

VIN K Spark Plug Wires Resistance
3280 - 4921 ohms per meter (1000 - 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.

       - Malfunctioning ignition control module.
       - Ignition System Wiring - Loose ignition module feed or ground connection, or damaged system wiring.
    • Remove spark plugs and check for the following conditions:
       - Fouled plugs
       - Cracks
       - Wear
       - Incorrect gap
       - Burned or damaged electrodes
       - Incorrect heat range
    • 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:
       - Compression
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Valve timing
       - Bent push rods
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in combustion chamber-Leaking valve seals. Refer to Oil Consumption Diagnosis .
    • For incorrect basic engine parts. Inspect the following parts:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.

Additional Checks
    • Check the exhaust system for a possible restriction in one or more of the following areas:
       - Damaged or collapsed pipes in the exhaust system
       - Heat distress or possible internal failure of the muffler
       - Possible plugged Three-Way catalytic converter
       - Loose, worn, or broken motor mounts
    • Refer to Restricted Exhaust System Check .
    • 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. Inspect the following items:
       - Incorrect routing of secondary ignition wires
       - Cracks, carbon tracking, or signs of arcing in the ignition coils
       - Casting flash in the intake and exhaust manifold passages