Symptom Definition

Engine runs unevenly at idle. If bad enough, vehicle will shake. Idle may vary in RPM. Engine idles at incorrect RPM.

Possible Cause & Correction Check the following:

• Ensure throttle linkage and/or TPS are not sticking or binding.
• Ensure initial ignition timing is properly set.
• Check engine idle speed, both base idle and ECM idle.
• Check Idle Air Control (IAC) system. Check for foreign material in IAC bore. See DIAGNOSTIC AIDS in CODE 35 chart in appropriate TESTS W/CODES article.
• Check for proper operation of EGR system.
• Check park/neutral switch circuit. Ensure park/neutral switch is properly adjusted.
• Check power steering pressure switch circuit.
• Check exhaust system for restrictions, such as a damaged or collapsed pipe, muffler or catalytic converter. See BASIC TESTING article.
• If rough idle only occurs when engine is hot, check PCV valve for proper operation, check evaporative emission control system, check for proper spark plug gap and engine compression.