Stalls/Rough, Unstable or Incorrect Idle
***UPDATED BY TSB #259005, #259002 #259201, #909102.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:
- 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 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: Incorrect for defective sensor or damaged wiring.
- BAROMETRIC PRESSURE (ALTITUDE) SENSOR: Defective.
Refer to COMPUTERIZED ENGINE CONTROLS.
FUEL SYSTEM
CHECK:
- FUEL INJECTOR BALANCE TEST: Poor spray pattern, clogged or sticking injector(s).
- FUEL PRESSURE: Check fuel pressure and volume from pump. Check pressure regulator (system pressure too low).
- 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).
- INJECTOR O-RINGS: Unmeasured air leaking into manifold at injectors (replace with new green "viton" O-rings).
- RESTRICTED AIR SUPPLY TO AIR SHROUDED INJECTORS: Rubber particles clogging or restricting air restrictor orifice to air shrouded injectors.
- SPEEDOMETER CONNECTIONS IN INSTRUMENT PANEL: Poor or broken solder connections to speedometer in instrument panel printed circuit board. See audi TSB #909102.
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.
- VACUUM LEAKS: Test for vacuum leaks at intake system.
- UNMEASURED AIR LEAKS: Check for air leaks in intake system between air flow sensor and engine.