Rough, Unstable, or Incorrect Idle, Stalling

ROUGH, UNSTABLE OR INCORRECT IDLE, STALLING

Definition - 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.

PRELIMINARY CHECKS
Perform the careful visual/physical checks.

SENSORS
- Check heated oxygen sensor (HO2S). Inspect sensor for silicon contamination from fuel, or use of improper RTV sealant. The sensor will have a white, powdery coating, and will result in a high but false signal voltage (rich exhaust indication). The ECM will then reduce the amount of fuel delivered to the engine, causing a severe driveability problem.
- Check TP sensor for a sticking throttle shaft or binding linkage.
- Check ECT sensor using a digital multimeter to measure resistance, compare ECT sensor resistance with ambient temperature on a cold engine. If ECT sensor resistance is greater than or less than ambient air temperature, check for high resistance in ECT sensor circuit or the sensor itself. Compare resistance value to "Diagnostic Aids" chart on DTC 14 or DTC 15 chart. DTC 15
- Check MAP sensor response and accuracy.

FUEL SYSTEM

NOTE: To determine if the condition is caused by a rich or lean system, the vehicle should be driven at the speed of the complaint. Monitoring HO2S voltage will help identify problem.
Lean - HO2S voltage below 450 mV
Rich - HO2S voltage above 450 mV

- Check for lean fuel injector not supplying enough fuel at idle.
- Check for fuel in pressure regulator hose. If present, replace fuel pressure regulator assembly.
- Check Evaporative Emission Control System, refer to Chart C-3. Chart C-3 Canister Purge Check
- Perform a cylinder compression check.
- Check for fuel injector leaking.

IGNITION SYSTEM
- Check Ignition System.
- Check ignition system for moisture, dust, cracks, burns, etc. Spray plug wires with a fine water mist to check for shorts.
- Check ignition wires for shorts or faulty insulation.

ENGINE MECHANICAL
- Check for broken motor mounts.
- Check for low compression.

ADDITIONAL CHECKS
- Check for vacuum leaks. They can cause higher than normal idle.
- Check IAC operation, refer to Chart C-2. Chart C-2 Idle Speed Control (ISC) Circuit
- Check ECM grounds for clean, tight, and proper routing.

NOTE: If problem exists with A/C "ON," check A/C system operation.

- Check for EGR "ON," while idling. It will cause roughness, stalling and hard starting, refer to Chart C-7. Chart C-7 EGR System Check
- Check battery cables and ground straps should be clean and secure.
- Check A/C refrigerant pressure too high.
- Check for overcharge or faulty dual pressure switch.
- Check PCV valve for proper operation by placing finger over inlet hole in PCV valve end several times with the engine idling. PCV valve should snap back. If not, replace PCV valve.