Rough Or Unstable Idle

Ensure idle RPM is correct.
Check air intake system for restrictions.
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 no vacuum leaks exist.
Check throttle position sensor operation.
Check for EGR system malfunction.
Check Engine Coolant Temperature (ECT) sensor operation.
Check Heated Oxygen Sensor (HO2S) voltage output.
Check for poor quality fuel.

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