Rough Or Unstable Idle

Use stethoscope to verify fuel injector operating noise.
Ensure fuel system pressure is correct.
Check EGR valve condition and operation.
Verify ignition timing is correct.
Check PCV valve.
Check airflow sensor.
Check coolant thermosensor for correct resistance.
Check idle air control valve function.
Check for cracks and poor connections (vacuum leaks) at airflow meter or throttle body.
Check O2 sensor for fluctuating 0.1-1.0 volt output while increasing and decreasing engine RPM.
Verify throttle position sensor has correct adjustment and resistance value.
Check purge valve function.
Check for poor fuel quality.