Common Causes:
* Faulty Idle Air Control (IAC) Valve: This valve regulates the amount of air entering the engine at idle. A dirty or malfunctioning IAC valve will often cause a high idle. Cleaning it (with appropriate cleaner) is often the first step; replacement is necessary if cleaning doesn't resolve the issue.
* Vacuum Leaks: Leaks in the vacuum system can disrupt the engine's control of idle speed. Check all vacuum hoses and connections for cracks, splits, or loose fittings.
* Throttle Position Sensor (TPS): A faulty TPS provides incorrect information to the engine's computer, leading to an irregular idle, which can manifest as a high idle.
* Mass Airflow Sensor (MAF) Sensor: Similar to the TPS, a dirty or malfunctioning MAF sensor provides inaccurate air-flow data, potentially resulting in a high idle. Cleaning the MAF sensor (with a MAF sensor cleaner ONLY – never use anything else) is often the first step. If cleaning doesn't work, replacement may be needed.
* Dirty Throttle Body: A buildup of carbon and deposits on the throttle body can interfere with its smooth operation and lead to a high idle. Cleaning the throttle body is often recommended as part of general maintenance or when diagnosing idle issues.
* Faulty PCV Valve: A clogged or malfunctioning Positive Crankcase Ventilation (PCV) valve can create excessive pressure in the crankcase, affecting the engine's breathing and potentially leading to a high idle.
* Engine Coolant Temperature (ECT) Sensor: An inaccurate ECT sensor can cause the engine to run at a richer air-fuel mixture at idle, increasing RPMs.
Less Common, but Possible Causes:
* Faulty Crankshaft Position Sensor (CKP): This sensor plays a critical role in engine timing, and a malfunction can lead to various issues, including high idle.
* Problems with the Engine Control Module (ECM): While less likely, a problem with the ECM itself could be responsible. This is a more serious and expensive repair.
* Air Leaks in the Intake Manifold: A crack or leak in the intake manifold will lead to unmetered air entering the engine, disrupting the idle mixture.
Troubleshooting Steps:
1. Check for obvious vacuum leaks: Visually inspect all hoses and connections.
2. Clean the IAC valve: This is often a quick and easy solution.
3. Clean the throttle body: Use a throttle body cleaner.
4. Clean the MAF sensor: Use a dedicated MAF sensor cleaner.
5. Check engine codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the ECM. These codes can pinpoint the likely source of the problem.
Important Note: If you are not comfortable working on your car, it's best to take it to a qualified mechanic for diagnosis and repair. Incorrect diagnosis or repair could damage your engine. Start with the easiest and cheapest solutions (visual inspection, cleaning the IAC and throttle body) before moving on to more expensive components.
