Stalls/Rough, Unstable or Incorrect Idle

SYMPTOM DESCRIPTION:
The engine runs unevenly at idle. If bad enough, the vehicle may shake. Also, the idle may vary in rpm (called "hunting"). Either condition may be severe enough to cause stalling. Engine idles at incorrect speed.




SYSTEM OR COMPONENT TO CHECK

COMPUTER INPUTS/OUTPUTS

CHECK:

- CRANKSHAFT POSITION SENSOR: Check for improperly positioned or damaged sensor, damaged wiring to control unit, missing reference pin (on flywheel) or accumulation of ferrous metal particles on sensor tip.
- ENGINE SPEED SENSOR: Check for improperly positioned or damaged sensor, damaged wiring to control unit: flywheel teeth damaged, missing or severely worn: accumulation of ferrous metal particles on sensor tip.
- IDLE STABILIZER VALVE: Control unit or wiring defective, defective or sticking valve.
- AIR MASS SENSOR: Defective sensor, or damaged wiring to control unit.
- OXYGEN SENSOR (O2) - Inspect sensor for sulfur or carbon contamination from fuel, or silicon contamination from use of improper RTV sealant. The sensor will have a white powdery coating and generate a high but false signal voltage (rich exhaust indication). The control unit will then reduce the amount of fuel delivered to the engine, causing a severe driveability problem.
- THROTTLE SWITCHES: Improper adjustment, shorted switch or damaged wiring to control unit.
- COOLANT TEMPERATURE SENSOR: Defective sensor, poor connection/corroded connector terminals or damaged wiring to control unit.

Refer to COMPUTERIZED ENGINE CONTROLS.




FUEL SYSTEM

CHECK:

- FUEL INJECTOR BALANCE TEST: Poor spray pattern; leaking, clogged or sticking injector.
- FUEL PRESSURE: Check fuel pressure and volume from pump. Check fuel pressure regulator (system pressure too high or too low).
- FUEL CONTAMINATION: Water or diesel in fuel.




IGNITION SYSTEM

CHECK:

- SPARK PLUGS: Wet with water or fuel, improperly gapped, heavy deposits, cracked insulators, or worn electrodes.
- SPARK PLUG WIRES: Damaged insulation, arcing to cylinder head or to other wires, secondary resistance too high.
- IGNITION DISTRIBUTOR, CAP, AND ROTOR: Check for cracks, arcing, moisture, rust deposits or high resistance through rotor. Check for defective hall sensor.
- IGNITION TIMING: Check distributor basic adjustment. See IGNITION TIMING under ADJUSTMENT PROCEDURES

Refer to IGNITION SYSTEMS.



ENGINE

CHECK:


- CRANKCASE AIR LEAKS: Check at dip stick seal, oil filler cap, front and rear engine oil seals.
- UNMEASURED AIR LEAKS: Check for air leaks in intake system between air mass sensor and engine.