* Vacuum Leaks: A vacuum leak anywhere in the intake system can cause a high idle. The engine is trying to compensate for lost air by letting in more air through the IAC, resulting in a higher idle. Check all vacuum hoses for cracks, holes, or loose connections. Pay close attention to the hoses connected to the brake booster, manifold absolute pressure (MAP) sensor, and the PCV valve.
* Throttle Position Sensor (TPS): A faulty TPS can send incorrect signals to the computer, causing it to adjust the idle incorrectly. A failing TPS often leads to erratic idle speeds, not just a consistently high one, but it's worth checking.
* Mass Air Flow (MAF) Sensor: This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can provide inaccurate readings, leading to incorrect fuel delivery and a high idle. Cleaning it (carefully, with MAF sensor cleaner) is often the first step, but replacement might be necessary.
* EGR Valve (Exhaust Gas Recirculation): A malfunctioning EGR valve can cause excessive exhaust gas to be recirculated, leading to a lean condition and a high idle in some cases. Check for sticking or blockage.
* Faulty IAC Valve (despite replacement): While you replaced the IAC, there's a small chance the new one is defective or not correctly installed. Double-check the installation and consider testing the new valve if possible.
* Computer Issues (PCM): The Powertrain Control Module (PCM) is the "brain" of the engine. A malfunctioning PCM can cause a wide array of problems, including incorrect idle control. This is less likely but possible.
* Fuel Pressure Regulator: A faulty fuel pressure regulator could lead to excessively rich fuel mixture, causing a higher idle speed to compensate.
* Throttle Body: A dirty or sticking throttle body can interfere with smooth idle operation. Cleaning the throttle body thoroughly might resolve the issue.
Troubleshooting Steps:
1. Check for vacuum leaks: This is the most common cause. Use a vacuum gauge or spray carburetor cleaner around suspected leak points while the engine is running and listen for a change in RPM.
2. Inspect and/or clean the MAF and TPS sensors: These are relatively easy to access and clean.
3. Check the EGR valve operation: Ensure it's not sticking or blocked.
4. Clean the throttle body: Use throttle body cleaner and carefully clean all passages.
5. Verify the correct installation of the new IAC valve: Ensure it's seated properly and all connections are secure.
6. Check fuel pressure: A fuel pressure gauge can help determine if the fuel pressure is within specifications.
If you've checked all of these and the problem persists, it's best to consult a qualified mechanic or use a professional OBD-II code reader to diagnose any stored trouble codes that could point to the specific issue. A code reader will often point you directly to the likely culprit.
