Rough Or Unstable Idle

Ensure idle RPM is correct.
Check air intake system for restrictions.
Check auxiliary air by-pass regulator.
Ensure fuel system pressure and volume are correct.
Check fuel injector electrical connections.
Use stethoscope to verify consistent operational noise coming from fuel injectors.
Ensure fuel injector spray pattern is correct.
Ensure sufficient secondary spark is available in all cylinders.
Ensure vacuum hose routing is correct and there are no vacuum leaks.
Check cold start valve for leaking (engine warm).
Check idle stabilizer operation.
Check throttle valve switch.
Check for EGR system malfunction (if equipped).
Check operation of coolant temperature sensor.
Check O2 sensor operation.
Check for distortion or cracks in fuel injector(s) plastic connecting flange.
Check for poor quality fuel containing insufficient cleaning additives.

NOTE: Prolonged use of poor quality fuel can cause injector clogging and carbon build-up on intake system.