GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 1999 🢒 Century (China) (CNG) 🢒 Engine 🢒 Engine Controls - 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
    • Check the fuel Pressure. Refer to Fuel System Pressure 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 Diagnostic Aids for DTC P0172.
    •  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.

Sensor/System
    •  Check for conditions which cause an incorrect idle speed.
       -  Throttle body tampering, excessive deposits, or damage. Refer to Fuel Supply Component Description.
       -  Restricted air intake system. Refer to Air Intake System Description .
       -  Large vacuum leak.
    •  Check Crankcase ventilation valve for proper operation. Refer to Crankcase Ventilation System Description .
    •  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.
    •  Check the Transaxle Range Switch input with the vehicle in drive and the gear selector in drive or overdrive.

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 Resistance
5000-7000ohms (5K -7Kohms).

       -  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 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 spark plugs and check 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 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 exhaust system for possible restrictions. Check for the following:
       -  Inspect exhaust system for damaged or collapsed pipes.
       -  Inspect mufflers for heat distress or possible internal failure.
       -  For possible plugged Catalytic Converter.
    •  Check for faulty 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 in RPM with little change in actual engine RPM change, indicates EMI is present. If a problem exists, check routing of secondary ignition wires, high voltage components (Near Ignition Control circuits).
    •  Check for faulty motor mounts. Refer to Engine Mechanical for inspection of mounts.
    •  Check for intake and exhaust manifold passages for casting flash. Refer to Engine Mechanical.