Rough Engine Idle

Check for vacuum leaks.
Check EGR system.
Ensure proper spark exists from ignition system.
Ensure ignition timing is correct.
Ensure fuel system performs properly (fuel pump, fuel filter, fuel injector and fuel pressure regulator).
Check engine compression, valve clearance, valve timing and timing belt (if equipped).
Check airflow meter or MAP sensor and wiring.
Check Engine Control Module (ECM) and wiring.
Ensure dipstick and oil filler cap are properly seated.