Vacuum Leaks: This is often the *most common* cause of a high idle. A leak in any part of the vacuum system can disrupt the engine's idle control. Check:
* Vacuum hoses: Inspect all vacuum hoses for cracks, holes, or loose connections. Pay close attention to those connected to the brake booster, the EGR valve, and the intake manifold.
* Intake manifold gasket: A leak here is a significant possibility. Look for signs of oil or vacuum leaks around the manifold.
* EGR valve: A faulty or stuck-open EGR valve can cause high idle. It can be cleaned or replaced.
Throttle Position Sensor (TPS): This sensor tells the computer the throttle's position. A faulty TPS can lead to an incorrect fuel mixture and a high idle.
Idle Air Control (IAC) Valve: This valve regulates the amount of air entering the engine at idle. A dirty or malfunctioning IAC valve is a common cause of high idle issues. It can often be cleaned, though replacement is sometimes necessary.
Mass Airflow Sensor (MAF) Sensor: While less likely to directly cause a *high* idle, a faulty MAF sensor can provide inaccurate readings to the computer, resulting in an incorrect air/fuel mixture and potentially contributing to idle problems (although it's more associated with rough idle or poor performance).
Computer Issues (PCM): While less frequent, a problem with the Powertrain Control Module (PCM) can cause various engine issues, including a high idle. This is usually diagnosed last after checking all other possibilities.
Throttle Body: A dirty or sticky throttle body can hinder the smooth operation of the throttle plate, resulting in a high idle. Cleaning the throttle body is often a good first step.
Other Potential Causes (Less Common):
* Faulty oxygen sensor: This affects the fuel mixture over time, but a high idle is less likely its primary symptom.
* Vacuum advance on the distributor (if applicable): While less common in later models, a malfunctioning vacuum advance could affect idling.
Troubleshooting Steps:
1. Check all vacuum hoses: This is the easiest and often most effective first step.
2. Clean the throttle body and IAC valve: This often resolves the issue, particularly if the engine is older and hasn't been maintained well.
3. Inspect the TPS: You may need a multimeter to check its voltage output.
4. Check the MAF sensor: A dirty MAF sensor can be cleaned with MAF sensor cleaner (follow instructions carefully). If cleaning doesn't help, it may need replacement.
5. Use a code reader: A code reader can diagnose any stored trouble codes in the PCM, pointing you toward the likely culprit. Even without a check engine light, codes may be stored.
If you're not comfortable working on your Jeep yourself, take it to a mechanic for diagnosis and repair. Improperly addressing these issues could damage your engine. Remember to always disconnect the negative battery terminal before working on any electrical components.
