GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2001 🢒 LeSabre 🢒 Engine 🢒 Engine Electrical 🢒 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 - Engine Controls .
    • Refer to Visual/Physical Symptoms - Engine Controls .
    • Search for bulletins.

Fuel System
    • Check the fuel pressure.
        Refer to Fuel System Diagnosis .
    • Check the fuel injectors. Refer to Fuel Injector Solenoid Coil Test
    • 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 DTC P0172 .
    • Check items that can cause an engine to run lean (long term fuel trim is significantly in the positive range). Refer to DTC P0171 .

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 Assembly Replacement .
       - Inspect for a large vacuum leak.
    • Check the crankcase ventilation valve for proper installation. Refer to Crankshaft Position Sensor Replacement .
    • Check the throttle position (TP) sensor and related wiring. Refer to DTC P0123 .
    • Check the exhaust gas recirculation (EGR) system for proper operation. Refer to DTC P0401 .
    • 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 Range Reference .

Ignition System
    • Check for proper ignition voltage output using the following steps:
       - Clip the J 26792 Spark Tester to engine ground.
       - Connect one end of a spark plug wire to the J 26792 . Connect the other end of the spark plug wire to the coil being tested.
       - Connect another spark plug wire between the remaining tower of the coil being tested and ground.
       - Crank the engine while observing the J 26792 . A spark should be observed.
       - Repeat the above steps for each coil.
    • If spark is not present at the plugs, check for the following conditions:
       - Check the coils for cracks, carbon tracking/arcing, or a secondary 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.

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 condition. 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.
       - Check the spark plug for cracks.

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

       - Check the spark plugs for carbon tracking on the porcelain and pitting on the plug wire contact.
       - Check the spark plugs for incorrect gap.
       - Check the spark plugs for burned or damaged electrodes. Refer to
       - 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.
    • Check for the following intermittent ignition system conditions.
        1. Check for an intermittent CKP Sync or CKP 18 X Signal.
        2. Check for an intermittent ignition feed circuit or sensor ground circuit to the crankshaft position sensor or the camshaft position (CMP) sensor.
        3. Check for an intermittent 3X reference signal.
        4. Check for an intermittent CMP signal.

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 excess oil in combustion chamber--Inspect for leaking valve seals.
    • Refer to the appropriate misfire procedures in Engine Mechanical. Inspect for incorrect engine parts, such as the following basic components:
       - The camshaft
       - The cylinder heads
       - The pistons, etc.

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 .
    • Check for a loose, worn, or broken motor mounts. 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.