Engine Difficult to Start under Normal Conditions

Ensure fuel system performs properly (fuel pump, fuel filter, fuel injector and fuel pressure regulator).
Ensure proper spark exists from ignition system.
Ensure ignition timing is correct.
Check STA signal at Engine Control Module (ECM).
Check engine compression, valve clearance, valve timing and timing belt (if equipped).
Check idle air control valve (if equipped) and wiring.
Check EGR system.
Check cold start injector system (if equipped).