Rough Or Unstable Idle

Check IAC valve for malfunction.
Ensure fuel system pressure is correct.
Ensure idle speed is correct.
Check air intake system for restrictions.
Use stethoscope to verify operational noise coming from fuel injectors.
Ensure sufficient secondary spark is available in all cylinders.
Verify vacuum hose routing is correct and that there are no vacuum leaks.
Ensure fuel system pressure and volume are correct.
Check for EGR system malfunction.
Check coolant temperature sensor operation.
Check intake air temperature sensor operation.
Verify throttle position sensor has correct adjustment and resistance value.
Check air conditioner switch operation.
Check inhibitor switch operation (automatic transmission).
Check IAC servo drive signal and operation.
Check airflow sensor voltage output.
Check barometric pressure sensor voltage output.
Check crank angle sensor operation.
Check vehicle speed sensor operation.
Check TDC sensor or cylinder No. 1 TDC sensor operation.
Check electrical connections at fuel injectors.