GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2000 🢒 GL7 (Export) 🢒 Engine 🢒 Engine Controls - 3.0L (LW9) 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Inspection/Test

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
    •  Refer to Intermittent Conditions .
    •  Search for bulletins.

Fuel System
    • Test the fuel pressure. Refer to Fuel System Pressure Test .
    • Test the fuel injectors. Refer to the following procedures:
      •  Fuel Injector Balance Test
    • Test for contaminated fuel. Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
    • Inspect for fuel in the pressure regulator vacuum hose.
    • Ensure each injector harness is connected to the correct injector and cylinder.
    •  Inspect for any items which may cause an engine to run rich, long term fuel trim is significantly in the negative range. Refer to Diagnostic Aids for DTC P0172.
    •  Inspect for any items which may cause an engine to run lean, long term fuel trim is significantly in the positive range. Refer to Diagnostic Aids for DTC P0171.

Sensor/System
    • Inspect for conditions which can cause an incorrect idle speed:
       -  Throttle body tampering, excessive deposits or damage. Refer to Fuel Metering System Component Description .
       -  A restricted air intake system
       -  A large vacuum leak
    • Inspect the crankcase ventilation valve for proper operation. Refer to Crankcase Ventilation System Description .
    • Inspect the TP sensor and the related wiring. Refer to DTC P0123 Throttle Position (TP) Sensor Circuit High Voltage .
    •  Monitor the 24X crankshaft position (CKP) and camshaft position (CMP) sensors on scan tool. If both are not responding, test the sensor feed circuit. Both sensors use a separate feed circuit but are internally connected to power. Test all 24X and CMP sensor circuits for intermittents. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
    •  Monitor the 3X parameter on the scan tool. If the 3X is not responding, inspect the 7X CKP sensor and circuits for intermittents. Inspect the ignition control (IC) circuit, bypass circuit, 3X reference high circuit, and the 3X reference low circuit for intermittents. If these circuits become open, or shorted may not set a DTC immediately, but are capable of causing driveability complaints. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
    •  Test the exhaust gas recirculation (EGR) system for proper operation.
    •  Inspect the Transaxle Range Switch input with the vehicle in drive and the gear selector in drive or overdrive.

Ignition System
    • Inspect for proper ignition voltage output using the following steps:
        1.  Attach the J 26792 spark tester to engine ground.
        2.  Connect the spark plug end of the spark plug wire to the J 26792 spark tester; leave the other end of the spark plug wire connected to the coil being tested.
        3.  Connect the spark plug end of the companion spark plug wire to ground.

The companion spark plug wire is the wire attached to the corresponding coil tower.

        4.  Crank the engine while observing the J 26792 spark tester; a spark should be observed.
        5.  Repeat the above steps for each coil.
    •  If spark is not present at the coils, inspect for the following conditions:
       -  Coils--Cracks, carbon tracking/arcing, or a resistance value outside the specified range.

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

       -  Spark Plug wires--signs of arcing, cross firing, cracks, carbon tracking, plug boot damage, pinched, improper routing, or a resistance value outside the specified range.

Spark Plug Wire Resistance
600ohms per foot (1 968ohms per meter).

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

       -  Defective ignition module.
       -  Ignition system wiring--Loose ignition module feed or ground connection, or damaged system wiring.
    •  Remove spark plugs and inspect for the following conditions:
       -  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
    • Check for the following engine mechanical concerns:
       - Low compression
       -  Sticking or leaking valves
       -  Worn camshaft lobes
       -  Valve timing
       -  Bent push rods
       -  Worn rocker arms
       -  Broken valve springs
       -  Excessive oil in combustion chamber and leaking valve seals
    • Inspect the following components for incorrect basic engine parts. Refer to Base Engine Misfire Diagnosis in Engine Mechanical:
       -  Camshaft
       -  Cylinder heads
       -  Pistons, etc.

Additional
    • Inspect the exhaust system for restrictions. Refer to Restricted Exhaust System Check :
       -  Inspect the exhaust system for damaged or collapsed pipes.
       -  Inspect the mufflers for heat distress or internal failure.
       - Inspect for a plugged 3-way catalytic converter.
    • Inspect for faulty motor mounts.
    •  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 in RPM with little change in the actual engine RPM indicates EMI is present. If a problem exists, inspect the routing of secondary ignition wires near the high voltage components .
    • Inspect the intake and exhaust manifold passages for casting flash. Refer to Base Engine Misfire Diagnosis in Engine Mechanical.