Rough, Unstable, or Incorrect Idle, Stalling

DEFINITION
Engine runs unevenly at idle. If severe, the engine or vehicle may shake. Engine idle speed may vary in RPM. Either condition may be severe enough to stall engine.

PRELIMINARY CHECKS
^ Perform the careful visual/physical checks as described in "PRELIMINARY CHECKS". Testing and Inspection
^ Check for vacuum leaks.
^ Check for Service Bulletin updates.
^ Inspect PCM grounds for being clean, tight and in their proper location.

FUEL SYSTEM

NOTE: Monitoring long term fuel trim may help identify the cause of the problem.

^ Long term fuel trim near 198 is a rich command compensating for a lean running engine. If the engine is very lean, a long term fuel trim of 198 may not fully compensate for the lean condition, resulting in driveability symptoms.
^ Long term fuel trim near 58 is a lean command compensating for a rich running engine. If the engine is very rich, a long term fuel trim of 58 may not fully compensate for the rich condition, resulting in driveability symptoms.
^ Check fuel injectors. Perform injector coil/balance test. Refer to "DIAGNOSTIC CHARTS/C-CHARTS/CHART C-2A FUEL INJECTOR TESTING". Chart C-2A Fuel Injector Balance Test
^ Check for fuel in pressure regulator hose. If fuel is present, replace regulator assembly.
^ Check Evaporative Emission (EVAP) control system for proper operation. Refer to "DIAGNOSTIC CHARTS/C-CHARTS/CHART C-3 EVAP CANISTER PURGE SOLENOID CHECK". Chart C-3 EVAP Canister Purge Solenoid Check
^ Check Oxygen Sensor (02S). The 02S should respond rapidly to the varying quantities of oxygen in the exhaust system. After the 02S has reached operational temperature, the voltage should change quickly, and can go as high as 1 volt and as low as 0.1 volt in less than a second. If this response time is slow or voltage is at a steady level, the 02S should be checked for silicone, glycol or other contamination. The 02S may have a white powdery coating (silicone contamination) or a green coating (glycol contamination) that would result in a false signal voltage to PCM, may lead to a driveability complaint.

IGNITION SYSTEM
^ Check ignition system secondary components for arcing, excessive resistance, cracked, worn, fouled, improperly gapped, or incorrect type spark plugs.
^ Check for ingition system misfire. Refer to "DIAGNOSTIC CHARTS/C-CHARTS/CHART C-4M ELECTRONIC IGNITION (EI) SYSTEM MISFIRE DIAGNOSIS. Chart C-4M Electronic Ignition System Misfire Diagnosis

ADDITIONAL CHECKS
^ Check MAP sensor. Refer to "DIAGNOSTIC CHARTS/C-CHARTS/CHART C-1D MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR OUTPUT CHECK". Chart C-1D Manifold Absolute Pressure (MAP) Output Check
^ Check throttle linkage for sticking or binding.
^ Check Park/Neutral Position (PNP) switch or trans range switch circuit. Refer to "DIAGNOSTIC CHARTS/C-CHARTS/CHART C-1A PARK NEUTRAL POSITION-TRANS RANGE SWITCH DIAGNOSIS". Chart C-1A Park/Neutral Switch Diagnosis A scan tool can also be used. Be sure tool indicates vehicle is in drive with gear selector in drive or overdrive (if equipped).
^ Check IAC valve operation. Refer to "DIAGNOSTIC CHARTS/C-CHARTS/CHART C-2C IDLE AIR CONTROL (IAC) VALVE CHECK". Chart C-2B Idle Air Control (IAC) System Check
^ Check A/C signal to PCM. Tech-1 scan tool should indicate A/C is being requested when ever A/C is selected. If problem exists with A/C "ON," check A/C system operation. Refer to "DIAGNOSTIC CHARTS/C-CHARTS/CHART C-10 A/C CLUTCH CONTROL CIRCUIT DIAGNOSIS". Chart C-10 A/C Clutch Control Circuit Diagnosis
^ Check for broken motor mounts.
^ Check alternator output voltage. Repair if less than 9 or more than 16 volts.
^ Check for intake valve deposits.