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, or missing reference pin (on flywheel).
- IDLE STABILIZER VALVE: Control unit or wiring defective, defective or sticking valve.
- AIR FLOW SENSOR POTENTIOMETER: Defective sensor, trim screw out of adjustment, or damaged wiring to control unit.
- OXYGEN SENSOR - Inspect sensor for silicon contamination from fuel, or 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 or wiring.




FUEL SYSTEM

CHECK:

- FUEL INJECTOR BALANCE TEST: Poor spray pattern, clogged or sticking injector.
- FUEL PRESSURE: Check fuel pressure and volume from pump.
- FUEL DISTRIBUTOR: Sticking control plunger, clogged fuel passages in fuel distributor body.
- FUEL CONTAMINATION: Water or diesel in fuel.
- AIR FLOW SENSOR: Sensor plate adjustments incorrect, basic air/fuel ratio incorrect (check idle CO content and control pressure).




IGNITION SYSTEM

CHECK:

- SECONDARY IGNITION SYSTEM: Secondary resistance too high.
- KNOCK SENSOR: Defective sensor.
- HALL SENSOR: Defective sensor or wiring.



ENGINE

CHECK:


- CRANKCASE AIR LEAKS: Check at dip stick seal, oil filler cap, front and rear engine oil seals.
- VACUUM LEAKS: Test for vacuum leaks at intake system.



Also see Audi TSB #258905, #259001.