GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2000 🢒 Regal (China) 🢒 Engine 🢒 Engine Controls - 2.5L (LB8) and 3.0L (L46) 🢒 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 Intermittent Conditions .
    •  Search for bulletins.

Fuel System
    • Inspect the fuel pressure. Refer to Fuel System Pressure Test .
    • Inspect 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/cylinder.
    • Inspect items which can cause an engine to run rich (long term fuel trim is significantly in the negative range). Refer to Diagnostic Aids in DTC P0172 Fuel Trim System Rich .
    • Inspect items that can cause an engine to run lean (long term fuel trim is significantly in the positive range). Refer to Diagnostic Aids in DTC P0171 Fuel Trim System Lean .

Sensor/System
    • Inspect for conditions which cause an incorrect idle speed.
       -  Throttle body tampering, excessive deposits or damage--Refer to Fuel Metering System Component Description .
       -  Restricted air intake system
       -  Large vacuum leak
    • Inspect the crankcase ventilation valve for proper operation. Refer to Crankcase Ventilation System Description .
    • Inspect the TP sensor and related wiring. Refer to DTC P0123 Throttle Position (TP) Sensor Circuit High Voltage .
    • Inspect the EGR system for proper operation. Refer to Exhaust Gas Recirculation (EGR) System Description .
    • 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:
       -  Clip the J 26792 spark tester to engine ground.
       -  Connect one end of a spark plug wire to the J 26792 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 J 26792 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, inspect for the following conditions:
       -  Coils--Cracks, carbon tracking/arcing or a resistance value outside the specified range.

Coil Resistance
5K-7K ohms (5,000-7,000 ohms).

       -  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
1 968 ohms per meter (600 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 faulty.

       -  Defective module
       -  Ignition system wiring--Loose ignition module feed or ground connection, or damaged system wiring.
    •  Remove the spark plugs and inspect for the following:
       -  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 engine mechanical for the following:
       -  Check 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
    •  For incorrect basic engine parts--Inspect the following:
       -  Camshaft
       -  Cylinder heads
       -  Pistons, etc.
    •  Refer to Base Engine Misfire Diagnosis in Engine Mechanical.

Additional Checks
    • Inspect the exhaust system for possible restrictions:
       -  Inspect the exhaust system for damaged or collapsed pipes.
       -  Inspect the mufflers for heat distress or possible internal failure.
       - Inspect for possible plugged three-way catalytic converter. Refer to Exhaust Leakage - Not OBD II in Engine Exhaust and Restricted Exhaust System Check .
    • Inspect for faulty motor mounts. Refer to Engine Mount Inspection in 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 in RPM with little change in actual engine RPM change, indicates EMI is present. If a problem exists, inspect routing of secondary ignition wires, high voltage components (near ignition control circuits).
    • Inspect for faulty motor mounts.
    • Inspect for intake and exhaust manifold passages for casting flash. Refer to Base Engine Misfire Diagnosis in Engine Mechanical.