Rough Or Unstable Idle

Check fuel pressure.
Check ignition timing.
Check for leaking fuel injector(s).
Check for restricted air filter.
Check Throttle Position (TP) Sensor operation.
Check for open EGR valve at idle.
Check PCV valve operation.
Check O2 sensor for silicone contamination from fuel or improper RTV sealant.
Check for fuel in fuel pressure regulator hose, indicating a faulty fuel pressure regulator.
Check knock sensor operation (if equipped).
Check for correct charging system voltage.
Check canister purge system operation.
Check Idle Air Control (IAC) system operation (if equipped).
Check MAP sensor (if equipped) operation.
Check for high A/C pressure due to overcharge or faulty pressure switch.
Check power steering pressure switch and circuit operation.
Ensure battery cables are clean and tight.
Ensure engine compression is correct.