GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 1997 🢒 Riviera 🢒 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 .
    • Check the injectors. Refer to Fuel Injector Solenoid Coil Test .
    • Check for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Test .
    • Check for fuel in the pressure regulator vacuum hose.
    • Ensure each injector harness is connected to the correct injector/cylinder. Refer to Engine Controls Schematics .
    • 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 .
       - Large vacuum leak.
    • Check the crankcase ventilation valve for proper installation. Refer to Positive Crankcase Ventilation Valve .
    • 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:
       - Coil(s) - 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
7000 ohms (7K ohms)

VIN K Spark Plug Wires Resistance
10000 ohms (10K ohms)

   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:
       - Check compression.
       - Sticking or leaking valves.
       - Worn camshaft lobe(s).
       - Valve timing.
       - Bent push rods.
       - Worn rocker arms.
       - Broken valve springs.
       - Excessive oil in combustion chamber-Leaking valve seals.
    • For incorrect basic engine parts. Inspect the following:
       - Camshaft
       - Cylinder heads
       - Pistons, etc.
    • Refer to Engine Mechanical for diagnosis procedures.

Additional Checks
    • Check the exhaust system for a possible restriction.
       - Inspect exhaust system for damaged or collapsed pipes.
       - Inspect mufflers for heat distress or possible internal failure.
       - For possible plugged Three-Way catalytic converter. Refer to Restricted Exhaust System Check and/or Exhaust System .
    • Check for loose, worn, or broken motor mounts. Refer to 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 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 Mechanical for inspection of mounts.
    • Check for intake and exhaust manifold passages for casting flash. Refer to Engine Mechanical.