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 and physical checks as described in Preliminary Checks. Testing and Inspection

SENSORS
Check: Oxygen Sensor (02S) - 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 (high 02S indication). The Engine Control Module (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 a digital multi-meter (J39200) should indicate less than 1.25 volts with throttle closed.

Check: Engine Coolant Temperature (ECT) sensor - Using a scan tool compare engine coolant temperature with Intake Air Temperature (IAT) sensor on a cold engine.

If coolant temperature reading is 5 degrees greater than or less than intake air temperature, check for high resistance in ECT circuit or sensor itself. Compare resistance value to chart under "Diagnostic Aids" in Diagnostic Trouble Code (DTC) 15. Powertrain Controls - ECM/PCM

Check: Manifold Absolute Pressure (MAP) sensor response and accuracy. Refer to Chart C-1D MAP Sensor Voltage Output Check. Chart C-1D Manifold Absolute Pressure (MAP) Output Check

FUEL SYSTEM

NOTE: To determine if the condition is caused by a rich or lean 02S signal, monitoring long term fuel trim will help identify the problem.

Lean - Long term fuel trim greater than 150. Refer to Diagnostic Aids in DTC 44. Powertrain Controls - ECM/PCM

Rich - Long term fuel trim less than 115. Refer to "Diagnostic Aids" in DTC 45. Powertrain Controls - ECM/PCM

Check: Evaporative Emission Control System. Refer to Chart C-3 EVAP Canister Purge Valve Check. Chart C-3 EVAP Canister Purge Valve Check

Check: Perform a cylinder compression check.

Check: For injector(s) leaking. Check fuel pressure. Refer to Chart A-7 Fuel System Diagnosis. Related Tests, Information and Procedures

IGNITION SYSTEM
Check: Ignition system. Refer to Distributor Ignition (DI) System. Ignition System

Check: Ignition timing. Refer to underhood "Vehicle Emission Control Information label."

ADDITIONAL CHECKS
Check: Vacuum leaks can cause higher than normal idle.

Check: IAC Operation - Refer to Chart C-2C Idle Air Control (IAC) Valve Check. Chart C-2C Idle Air Control (IAC) System Check

Check: ECM grounds for clean, tight, and properly routing.

Check: PNP switch circuit. Refer to Chart C-1A Park/Neutral Position (PNP) Switch Diagnosis, or use scan tool, and be sure tool indicates vehicle is in drive with gear selector in drive or overdrive. Chart C-1A Park/Neutral Switch Diagnosis

NOTE: 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 A/C Clutch Circuit Diagnosis. Chart C-10 A/C Clutch Control Circuit Diagnosis

Check: EGR "ON," while idling, will cause roughness, stalling, and hard starting. Refer to Chart C-7 Exhaust Gas Recirculation (EGR) Check. Chart C-7 EGR System Check

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: 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.

Check: For low compression.