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.

SENSORS
^ Check the Heated Oxygen Sensor (HO2S) - Inspect sensor for silicon contamination from fuel, or use of improper sealant (never use sealant on any waterproof connectors). 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.
^ Check the 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 0.85 volt with throttle closed.
^ Check the 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 Procedures / Diagnostic Charts / Code 15 Coolant Sensor Circuit. Testing and Inspection
^ Check the Manifold Absolute Pressure (MAP) sensor response and accuracy, refer to Procedures / Diagnostic Charts / C Charts / Manifold Absolute Pressure (MAP) Output Check. Testing and Inspection

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 Procedures / Diagnostic Charts / Code 44 Oxygen Sensor Circuit, "Diagnostic Aids". Testing and Inspection
Rich - Long term fuel trim less than 115. Refer to Procedures / Diagnostic Charts / Code 45 Oxygen Sensor Circuit, "Diagnostic Aids". Testing and Inspection
^ Evaporative emission control system. Refer to Procedures / Diagnostic Charts / C Charts / Canister Purge Solenoid. Testing and Inspection
^ Perform a cylinder compression check.
^ Check for injector leakage, check fuel pressure. Refer to Procedures / Diagnostic Charts / Chart A-6 Fuel System Diagnosis. Testing and Inspection

IGNITION SYSTEM
^ Check Ignition system and timing, refer to Procedures / Diagnostic Charts / C Charts / Distributor Ignition System Check. Testing and Inspection

ADDITIONAL CHECKS
^ Check for vacuum leaks can cause higher than normal idle and low Idle Air Control (IAC) counts.
^ Check IAC operation, refer to Procedures / Diagnostic Charts / Code 35 Idle Air Control (IAC) System. Testing and Inspection
^ Check control module grounds for clean, tight, and proper routing.
^ Check the Trans Range (TR) pressure switch assembly operation. Refer to Procedures / Diagnostic Charts / C Charts / Transmission Range Pressure Switch Circuit Check. Testing and Inspection
^ Use scan tool to determine if control module is receiving A/C request signal. Whenever A/C is selected, refer to Procedures / Diagnostic Charts / C Charts / A/C Request Signal Diagnosis. Testing and Inspection
If problem exists with A/C "ON," check A/C system operation.
^ Check for Exhaust Gas Recirculation (EGR) "ON," while idling, will cause roughness, stalling and hard starting. Refer to Procedures / Diagnostic Charts / C Charts / EGR System Check. Testing and Inspection
^ Check the 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 the IAC valve. It will not move if system voltage is below 9 or greater than 16 volts.
^ Check the A/C refrigerant pressure for too high or faulty high pressure switch.
^ Check the 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 Perform a cylinder compression check.
^ Check For incorrect camshaft, worn camshaft or weak valve springs.