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
^ Refer to Important Preliminary Checks.
Testing and Inspection
^ Service bulletins for updates.

SENSORS
CHECK: The Oxygen Sensor (02S) should respond quickly to different throttle positions, if it does not, check the 02S 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 control module will then reduce the amount of fuel delivered to the engine, causing a severe driveability problem. Also, check for green (glycol) contamination or cracking.
Throttle Position (TP) sensor - If a sticking throttle shaft or binding linkage causes a high TP sensor voltage open throttle indication, the control module will not control idle. Monitor TP sensor voltage. A scan tool and/or voltmeter should read less than 1.25 volt with throttle closed.
Engine Coolant Temperature (ECT) sensor - Using a scan tool, compare engine coolant temperature with ambient temperature on a cold engine.
- If engine coolant temperature reads 5 degrees greater than or less than ambient air temperature. Check for high resistance in coolant sensor circuit or sensor itself, refer to DTC 15 Engine Coolant Temperature Sensor High - Diagnostic Aids.
Powertrain Controls - ECM/PCM
MAP sensor response and accuracy. Refer to Manifold Absolute Pressure Output Check.
MAP Sensor Output Diagnosis

FUEL SYSTEM
CHECK: 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 fuel trim will help identify problem.
Lean - Long term fuel trim greater than 150, refer to DTC 44 Lean Exhaust - Diagnostic Aids.
Powertrain Controls - ECM/PCM
Rich - Long term fuel trim less than 115, refer to DTC 45 Rich Exhaust - Diagnostic Aids.
Powertrain Controls - ECM/PCM
For fuel in pressure regulator vacuum hose. If fuel is present, replace regulator assembly.
Evaporative emission control system.
Perform a cylinder compression check.
For injector(s) leaking. Check fuel pressure. Refer to Chart A-6 Fuel System Diagnosis.
Related Tests, Information and Procedures

IGNITION SYSTEM
CHECK: Ignition voltage output using spark tester (ST 125) and ignition timing.
Spark plugs. Remove spark plugs, check for wet plugs, cracks, wear, improper gap, burned electrodes, blistered insulators, or heavy deposits. Repair or replace as necessary. Also, check spark plug wires.
Spark plug cables by connecting ohmmeter to ends of each cable in question. If meter reads over 30,000 ohms, replace cable(s).

ADDITIONAL CHECKS
CHECK: Vacuum leaks can cause higher than normal idle and low IAC counts.
IAC operation. Refer to DTC 35 Idle Air Control Error.

Control module grounds for clean, tight, and proper routing.
Transmission Range (TR) pressure switch assembly.
Use scan tool to determine if control module is receiving A/C request signal. If problem exists with A/C "ON," check A/C system operation.
EGR "ON" while idling, will cause roughness, stalling and hard starting.
Battery cables and ground straps should be clean and secure. Erratic voltage will cause IAC to change its position, resulting in poor idle quality.
IAC valve will not move if system voltage is below 9 or greater than 16 volts.
A/C refrigerant pressure too high or faulty high pressure switch.
Crankcase ventilation valve for proper operation by placing finger over inlet hole in valve end several times. Valve should snap back. If not, replace valve.

ENGINE MECHANICAL
CHECK: For broken motor mounts, proper valve timing, low compression, bent pushrods, worn rocker arms, broken or weak valve springs, worn camshaft lobes.