* Idle Air Control (IAC) Valve: This is the most common suspect. The IAC valve regulates air flow at idle. If it's sticking, dirty, or has a faulty electrical connection, it can cause a high idle. Restarting the car sometimes allows it to reset or temporarily overcome the sticking issue.
* Throttle Position Sensor (TPS): This sensor tells the computer the throttle's position. A faulty TPS can send incorrect signals, leading to a high idle. Again, a restart might temporarily clear a glitch.
* Mass Airflow Sensor (MAF) Sensor: This sensor measures the amount of air entering the engine. A dirty or malfunctioning MAF sensor can cause inaccurate readings, resulting in a high idle. Similar to the TPS, a restart might reset a temporary problem.
* Vacuum Leaks: A vacuum leak can cause the engine to take in more air than it should, leading to a higher idle. These can be hard to find, but a visual inspection of all vacuum lines and hoses is a good starting point. A leak might be more likely to persist after a restart, but some might temporarily seal themselves under certain conditions.
* Intermittent Electrical Connection: A loose or corroded connection in the wiring harness to any of the above sensors (or even the computer itself) could cause intermittent high idle. Vibration could temporarily disrupt the connection, which a restart might resolve.
* Computer (ECM) Issues: While less likely, a problem with the engine control module (ECM) could also be causing the erratic idle. This is more complex to diagnose.
Troubleshooting Steps:
1. Visual Inspection: Start with a visual inspection of all vacuum lines, hoses, and electrical connections related to the IAC, TPS, and MAF sensors. Look for cracks, loose connections, or corrosion.
2. Clean the IAC Valve: This is often a quick and easy fix. Remove the valve, clean it with carburetor cleaner, and reinstall it.
3. Check the TPS and MAF Sensor: You can test these with a multimeter, though this requires some technical knowledge and a wiring diagram. Cleaning the MAF sensor (carefully!) with MAF sensor cleaner can also help.
4. Check for Vacuum Leaks: Use a vacuum gauge or listen carefully around the intake manifold for hissing sounds while the engine is running.
5. Advanced Diagnostics: If the problem persists, you'll likely need a scan tool (OBD-II code reader) to check for diagnostic trouble codes (DTCs). These codes can help pinpoint the faulty component.
If you're not comfortable working on your car's engine, it's best to take it to a mechanic for diagnosis and repair. The intermittent nature of the problem makes it more challenging to pinpoint the cause without proper tools and expertise.
