Vacuum Leaks:
* Everywhere! A vacuum leak is a very common cause of high idle. Check all vacuum lines for cracks, holes, or loose connections. Pay close attention to the lines connected to the brake booster, PCV valve, and the intake manifold. A small leak can significantly affect idle. Use a vacuum gauge to test for leaks; a significant drop in vacuum indicates a problem.
* Intake Manifold Gasket: A leaking intake manifold gasket is a more serious issue. It's harder to detect visually but often causes irregular idling and performance problems.
Throttle Position Sensor (TPS):
* The TPS tells the computer the throttle's position. A faulty TPS can cause incorrect fuel delivery and lead to high idle. It's often overlooked, even if the IAC is replaced. Test its voltage output with a multimeter.
EGR Valve (Exhaust Gas Recirculation):
* A sticking or faulty EGR valve can introduce exhaust gases into the intake, affecting the air/fuel mixture and causing high idle. Try temporarily disconnecting it (be aware of potential emissions issues) to see if it improves the idle. If it does, replace the EGR valve.
Computer/PCM Issues:
* The Powertrain Control Module (PCM) could be faulty or have a programming issue. This is less likely but possible after ruling out other causes. A scan tool capable of reading PCM codes and live data is needed to diagnose this properly.
Other Sensors:
* Coolant Temperature Sensor (CTS): An inaccurate CTS can cause the PCM to compensate incorrectly for engine temperature, leading to high idle, especially when the engine is cold.
* Oxygen Sensor(s): While less likely to directly cause high idle, faulty O2 sensors can affect the air/fuel mixture long-term which can lead to compensation and high idle.
Troubleshooting Steps:
1. Thorough Vacuum Leak Inspection: This is the most likely culprit after ruling out the IAC and MAP. Start by visually inspecting all vacuum lines and connections. Then, consider using a vacuum gauge for a more definitive test.
2. TPS Test: Use a multimeter to test the voltage output of the TPS. Consult your vehicle's repair manual for the correct specifications.
3. EGR Valve Test (Temporary Disconnection): Disconnect the EGR valve temporarily to see if the idle improves.
4. OBD-II Scan: Get a diagnostic code scan from your vehicle's OBD-II port. This can reveal any stored trouble codes that point towards a specific sensor or system issue.
5. Professional Diagnosis: If you've exhausted the above steps and still have a high idle, it's time to consult a qualified mechanic with experience in diagnosing fuel injection systems. They'll have the tools and expertise to pinpoint the problem.
Remember safety first. Disconnect the battery negative terminal before working on any electrical components. Consult a repair manual for your specific vehicle for detailed diagrams and specifications.
