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 coolant temperature sensor and wiring.
Check idle air control valve (if equipped) and wiring.
Check airflow meter or vacuum sensor and wiring.
Check Electronic Control Module (ECM) and wiring.