Vacuum System Problems:
* Vacuum Leaks: A leak in the vacuum lines leading to the throttle body or other components can cause a high idle. These lines are prone to cracking and deterioration with age.
* Faulty Vacuum Switch: Various vacuum switches control different aspects of the engine's operation. A malfunctioning switch can disrupt the idle system.
* Blocked PCV Valve: A Positive Crankcase Ventilation (PCV) valve that's clogged can restrict airflow and disrupt the vacuum balance, leading to high idle.
Throttle Body Issues:
* Dirty Throttle Body: A dirty throttle body can prevent the throttle plate from closing completely, resulting in a high idle. Cleaning it with throttle body cleaner is often a simple fix.
* Faulty Throttle Position Sensor (TPS): This sensor tells the computer the throttle position. A faulty TPS can send incorrect signals, causing the engine to idle high.
* Sticking Throttle Cable/ Linkage: A cable or linkage that's sticking can prevent the throttle from closing all the way.
Other Potential Causes:
* Idle Air Control (IAC) Valve: The IAC valve controls the amount of air entering the engine at idle. A malfunctioning IAC valve is a common cause of high idle. It might be dirty or worn out.
* Faulty Temperature Sensor: An inaccurate coolant temperature sensor can cause the engine computer to adjust the idle incorrectly.
* EGR Valve: The Exhaust Gas Recirculation (EGR) valve can stick open or malfunction, contributing to a high idle.
* Electrical Problems: A short circuit or faulty wiring in the engine control system can also cause a high idle.
* Air Intake Restriction: A severely clogged air filter will lean out the mixture, causing the engine to compensate by increasing the idle speed. Less likely to cause a *high* idle, but possible if the filter is extremely clogged.
Troubleshooting Steps:
1. Check for vacuum leaks: Visually inspect all vacuum lines for cracks or loose connections. You can use a vacuum gauge to pinpoint leaks.
2. Clean the throttle body: This is a relatively easy task and should be done as a first step.
3. Inspect the IAC valve: See if it's dirty or sticking. Sometimes cleaning it can solve the problem.
4. Check the TPS: You'll likely need a multimeter to test the voltage output of the TPS.
5. Test the coolant temperature sensor: Again, a multimeter will be needed.
Important Note: Because there are so many possibilities, diagnosing the precise cause may require some mechanical aptitude and the use of diagnostic tools. If you're not comfortable working on your vehicle yourself, it's best to take it to a qualified mechanic for diagnosis and repair. Ignoring a high idle can lead to further engine damage.
