* Idle Air Control (IAC) Valve: This valve regulates the amount of air entering the engine at idle. A dirty or malfunctioning IAC can cause rough idling, stalling, or skipping. Cleaning it (with appropriate carb cleaner) is a relatively easy first step. If cleaning doesn't work, replacement might be needed.
* Throttle Position Sensor (TPS): This sensor tells the computer the throttle's position. A faulty TPS can lead to poor idle and hesitation. Testing its voltage output with a multimeter is crucial for diagnosis.
* Mass Airflow Sensor (MAF) Sensor: While less likely to cause a problem *only* at idle, a dirty or failing MAF sensor can impact the air/fuel mixture, potentially leading to a rough idle. Cleaning the MAF sensor (with MAF sensor cleaner – *never* use regular cleaner) is worth trying.
* Vacuum Leaks: Small vacuum leaks can significantly affect idle. Inspect all vacuum hoses and connections for cracks, holes, or loose fittings. Pay close attention to the PCV system. A hissing sound can indicate a leak.
* Ignition System (Beyond Wires and Plugs): While you replaced the wires and plugs, other ignition components could be at fault:
* Distributor Cap and Rotor (if applicable): Check for cracks, arcing, or corrosion.
* Ignition Coil: A weak coil might struggle to provide sufficient spark at low RPMs.
* Ignition Control Module (ICM): This module controls the ignition timing; a failing ICM can cause erratic idle.
* Fuel System:
* Fuel Injectors: A clogged or faulty injector can cause a rough idle. This is harder to diagnose without specialized tools.
* Fuel Pressure Regulator: A malfunctioning regulator could deliver inconsistent fuel pressure.
* Computer Issues (PCM): While less common, a failing PCM can cause a variety of driveability issues. This is usually diagnosed by a professional mechanic using scan tools.
Troubleshooting Steps:
1. Clean the IAC valve. This is the easiest and cheapest first step.
2. Check for vacuum leaks. Carefully inspect all vacuum hoses.
3. Clean the MAF sensor (using MAF sensor cleaner).
4. Inspect the distributor cap and rotor (if equipped).
5. Have a mechanic check the TPS and fuel pressure. These require testing tools.
If you've tried these steps and the problem persists, it's best to take the car to a qualified mechanic. They'll have the diagnostic tools (scan tool) needed to pinpoint the exact cause. Mentioning the symptoms (rough idle, smooth high speed) will help them focus their diagnostic efforts.
