Here's a breakdown of how to approach this, emphasizing the importance of diagnostics first:
1. Diagnostics are Key: Before making any adjustments, you *must* determine *why* the idle speed is high. A high idle isn't just a single problem; it's a symptom. Possible causes include:
* Vacuum Leaks: Check all vacuum hoses for cracks, splits, or disconnections. A leak in the system can cause a high idle. Pay close attention to hoses connected to the brake booster, intake manifold, and the various emissions components.
* Throttle Position Sensor (TPS): A faulty TPS can send inaccurate signals to the PCM (Powertrain Control Module), resulting in an increased idle speed. A scan tool can read the TPS voltage to check for proper operation.
* Idle Air Control (IAC) Valve: This valve regulates air entering the engine at idle. A dirty or malfunctioning IAC valve can cause a high idle. Cleaning it (with appropriate cleaner) might help, but replacement might be necessary.
* Mass Airflow Sensor (MAF) Sensor: A dirty or faulty MAF sensor can also cause the computer to compensate by raising the idle. Cleaning it (gently!) or replacing it may resolve the issue.
* Engine Coolant Temperature Sensor (ECT): A faulty ECT can lead to incorrect fuel mixture calculations, potentially resulting in a high idle, especially when the engine is cold.
* PCM Issues: In rare cases, a problem with the PCM itself could be the culprit. This is less likely, but a possibility.
2. Scanning the Codes: Get your Jeep's diagnostic trouble codes (DTCs) read using a scan tool (OBD-II scanner). Auto parts stores often offer this service for free. The codes will point towards the most likely cause.
3. Addressing the Underlying Problem: Once you've identified the problem (via diagnostics and inspection), address *that* problem. For example:
* Vacuum Leak: Repair or replace the leaking hose.
* Faulty Sensor: Replace the faulty sensor (TPS, IAC, MAF, ECT).
* Dirty Sensor: Clean the sensor (TPS, MAF) carefully with MAF sensor cleaner. *Do not use anything else.*
* IAC Valve: Clean the IAC valve with appropriate cleaner or replace it.
4. (Last Resort) Idle Speed Adjustment (Proceed with Caution): If *all* else fails, and your Jeep has a manually adjustable idle speed screw (though less likely on a 1999), you might find a small screw on the throttle body. ONLY adjust this *after* you've addressed all other potential problems. Turning it slightly *clockwise* usually lowers the idle, but do it in tiny increments and check your idle speed after each adjustment.
Important Note: Incorrectly adjusting the idle speed screw can damage your engine or cause other problems. It's far better to diagnose the underlying issue and fix that, rather than just attempting to tweak the idle speed. If you're not comfortable doing this yourself, take your Jeep to a qualified mechanic. They have the tools and expertise to properly diagnose and repair the problem.
