GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2000 🢒 Grand Prix 🢒 Body and Accessories 🢒 Wiring Systems 🢒 Rough, Unstable, or Incorrect Idle and Stalling

Checks

Action

Definition: The engine runs unevenly during idle. If the condition is severe, the engine or the vehicle may shake. The 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.

Fuel System
    • Test the fuel pressure. Refer to Fuel System Pressure Test (VIN K) . Refer to Fuel System Pressure Test (VIN 1) .
    • Test the fuel injectors using the following tests:
       -  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 that each injector harness is connected to the correct injector and cylinder.
    • Test for conditions which can cause the engine to run rich, indicating that the long-term fuel trim is significantly in the negative range. Refer to DTC P0172 Fuel Trim System Rich .
    • Inspect for conditions that can cause an engine to run lean, indicating that the long-term fuel trim is significantly in the positive range. Refer to DTC P0171 Fuel Trim System Lean .

Sensor/System
    • Inspect for conditions which can cause an incorrect idle speed.
       - Check for throttle body tampering, for excessive deposits, or for damage. Refer to Fuel Metering System Component Description .
       - Check for a restricted air intake system. Refer to Air Intake System Description .
       - Check for a large vacuum leak.
    • Test the crankcase ventilation valve for proper operation. Refer to Crankcase Ventilation System Description .
    • Test the TP sensor and the related wiring. Refer to DTC P0123 Throttle Position (TP) Sensor Circuit High Voltage .
    • Test the EGR system for proper operation. Refer to Exhaust Gas Recirculation (EGR) System Description .
    • Test the transaxle range switch input with the vehicle in drive, and with the gear selector in drive or overdrive.

Ignition System

Test for proper ignition voltage output using the following steps:

  1. Clip the spark tester J 26792 to engine ground.
  2. Connect one end of a spark plug wire to the spark tester, and leave the other end of the spark plug wire to the coil being tested.
  3. Connect one end of another spark plug wire to the other coil tower, and connect the other end of the spark plug wire to ground.
  4. Crank the engine while you observe the spark test. You should observe a crisp blue spark.
  5. Repeat the above steps for each coil.
  6. If no spark is present at the coils, check for the following conditions:
    7.  - The coils--Inspect for cracks, for carbon tracking or arcing, or for a secondary resistance value outside the specified range.

    Coil Resistance
    5,000-8,000 ohms (5K-8K ohms)

     - The 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
    600 ohms per ft.

       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.

     - The 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.
     - A defective module.
     - The ignition system wiring--Inspect for loose ignition module feed or ground connection, or damaged system wiring.
  7. Remove spark plugs and inspect for the following conditions:
    8.  - Fouled plugs
     - Cracks
     - Wear
     - Improper gap
     - Burned or damaged electrodes
     - Improper heat range or reach

If the spark plugs are gas or oil fouled, you must determine the cause of the fouling before you replace the spark plugs.

Engine Mechanical
    • Test and inspect engine mechanical components for the following conditions:
       - Improper test compression
       - Sticking or leaking valves
       - Worn camshaft lobes
       - Improper valve timing
       - Bent push rods
       - Worn rocker arms
       - Broken valve springs
       - Excessive oil in the combustion chamber that is caused by leaking valve seals
    • Inspect for incorrect basic engine components. Inspect the following components:
       - The camshaft
       - The cylinder heads
       - The pistons
       - Other engine components, as applicable

Additional Checks
    • Test the exhaust system for possible restrictions. Inspect for the following conditions:
       - Damaged or collapsed pipes.
       - Heat distress or possible internal failure in the muffler
       - A plugged 3-way catalytic converter. Refer to Restricted Exhaust System Check in Engine Exhaust.
    • Inspect for faulty motor mounts.
    • Electromagnetic interference (EMI) on the reference circuit can cause an engine miss condition. You can usually detect EMI by monitoring the engine RPM with a scan tool. A sudden increase in the RPM, with little change in actual engine RPM change indicates that EMI is present. If a problem exists, inspect the routing of secondary ignition wires and the high voltage components that are near the ignition control circuits.
    • Inspect the intake and the exhaust manifold passages for casting flash. Refer to Intake Manifold Cleaning and Inspection refer to in Engine Mechanical.