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 "Visual and Physical Checks" as described in Preliminary Checks. Testing and Inspection

SENSORS
CHECK: Heated Oxygen Sensor (HO2S) 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 Engine Control Module (ECM) will then reduce the amount of fuel delivered to the engine, causing a severe driveability malfunction.

CHECK: Throttle Position (TP) sensor for a sticking throttle shaft or binding linkage or misadjusted. Refer to Throttle Position Sensor / Adjustments for adjustment procedures.

CHECK: Engine Coolant Temperature (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 sensor itself. Compare ECT sensor resistance value to those in Engine Coolant Temperature Sensor / Specifications.

CHECK: Manifold Absolute Pressure (MAP) sensor response and accuracy.

FUEL SYSTEM

NOTICE: 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 with Tech 1 scan tool will help identify a malfunction.

Lean - HO2S voltage below 450 mV.
Rich - HO2S voltage above 450 mV.

CHECK: Lean fuel injector not supplying enough fuel at idle.
CHECK: For fuel in pressure regulator hose. If present, replace regulator assembly.
CHECK: Evaporative Emission (EVAP) control system.
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: Electric load signal.
CHECK: Vacuum leaks can cause higher than normal idle.
CHECK: ISC motor operation.
CHECK: ECM grounds for clean, tight, and properly routing.
CHECK: RD2L signal.

NOTICE: If malfunction exists with A/C "ON," check A/C system operation.

CHECK: EGR "ON," while idling, will cause roughness, stalling and hard starting.
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.