Rough, Unstable, or Incorrect Idle, Stalling
ROUGH, UNSTABLE OR INCORRECT IDLE, STALLINGDefinition: 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 as described at start of "Symptoms".
ADDITIONAL CHECKS
^ CHECK: Vacuum leaks can cause higher than normal idle.
^ CHECK: IAC operation - Use CHART C-2C.
^ CHECK: ECM grounds for clean, tight, and properly routing
^ CHECK: PNP switch circuit. Use CHART C-1A, or use scan tool, and be sure tool indicates vehicle is in drive with gear selector in drive or overdrive.
NOTICE: Use scan tool to determine if ECM is receiving A/C request signal. If problem exists with A/C "ON," check A/C system operation CHART C-10.
^ CHECK: EGR "ON," while idling, will cause roughness, stalling and hard starting. Use CHART C-7.
^ CHECK: Battery cables and ground straps should be clean and secure. Erratic voltage will cause IAC to change its position, resulting in poor idle quality.
^ CHECK: IAC valve will not move, if system voltage
is below 9 or greater than 16 volts.
CHECK: A/C refrigerant pressure too high.
^ CHECK: For overcharge or faulty high pressure switch.
^ CHECK: PCV valve for proper operation by placing finger over inlet hole in valve end several
times with the engine idling. Valve should snap back. If not, replace valve.
SENSORS
^ CHECK: Oxygen Sensor (025) - 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: Throttle Position (TP) sensor - If a sticking throttle shaft or binding linkage causes a high TP sensor voltage (open throttle indication, the ECM will not control idle. Monitor TP sensor voltage. A scan tool and/or voltmeter should read less than 1.25 volts with throttle closed.
^ CHECK: Engine Coolant Temperature (ECT) sensor - Using a scan tool compare engine coolant temperature with ambient temperature on a cold engine.
- If engine coolant temperature reading is 5 degrees greater than or less than ambient air temperature. Check for high resistance in coolant sensor circuit or sensor itself. Compare resistance value to "Diagnostics Aids" chart on DTC 15 chart.
^ CHECK: MAP sensor response and accuracy - Refer to CHART C-1D, MAP voltage output check.
ENGINE MECHANICAL
^ CHECK: For broken motor mounts.
^ CHECK: For low compression
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 LT fuel trim will help identify problem.
Lean - LT fuel trim greater than 150. Refer to "Diagnostic Aids" in DTC 44.
Rich - LT fuel trim less than 115. Refer to "Diagnostic Aids" in DTC 45.
^ CHECK: For fuel in pressure regulator hose. If present, replace regulator assembly.
^ CHECK: Evaporative Emission (EVAP) System.
^ CHECK: Perform a cylinder compression check.
^ CHECK: For injector(s) leaking. Check fuel pressure
IGNITION SYSTEM
^ CHECK: Ignition system. Refer to "Distributor Ignition (DI),"
^ 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.