* Throttle Position Sensor (TPS): A faulty TPS can send incorrect signals to the engine control unit (ECU), leading to an elevated idle. The ECU may compensate by increasing the idle speed. This is a common culprit.
* Idle Air Bypass (IAB) Valve: Some vehicles, even from that era, might have an IAB in addition to or instead of a solely functioning IAC. Ensure this secondary system (if present) is functioning correctly.
* Mass Air Flow (MAF) Sensor: A dirty or malfunctioning MAF sensor provides inaccurate air flow readings, causing the ECU to adjust the idle incorrectly.
* Engine Coolant Temperature (ECT) Sensor: An inaccurate ECT sensor reading can lead to improper fuel mixture and idle speed adjustments. A faulty sensor might cause the ECU to think the engine is cold, leading to a higher idle.
* Faulty ECU: While less likely, a problem within the ECU itself could be responsible. This is a more difficult diagnosis and often requires professional help.
* Throttle Body: A dirty or sticky throttle body can hinder its operation and cause a high idle. Cleaning the throttle body (carefully, following proper procedures) might resolve the issue.
* Vacuum lines (missed one): While you've checked for vacuum leaks, there might be a very small or hard-to-spot leak you missed, especially if the lines are brittle or cracked.
* PCV Valve: A clogged Positive Crankcase Ventilation (PCV) valve can build up pressure in the crankcase, affecting the engine's performance and idle speed.
* Faulty or worn-out ignition components: Worn spark plugs, wires, or distributor cap can lead to erratic engine behavior, potentially causing a higher idle.
Troubleshooting Steps:
1. Check for diagnostic trouble codes (DTCs): Use an OBD-I code reader (your car likely only supports OBD-I) to see if any codes are stored in the ECU. This can give you a clue about the problem.
2. Inspect and clean the throttle body: Clean it thoroughly with throttle body cleaner, ensuring you follow the proper procedure and protect any sensors.
3. Test the TPS: A multimeter can be used to test the TPS voltage output. This requires looking up the correct specifications for your vehicle.
4. Check the MAF sensor: Inspect the MAF sensor for dirt or debris. Cleaning it (carefully, often with MAF sensor cleaner) might help.
5. Test the ECT sensor: Similar to the TPS, the ECT sensor can be tested with a multimeter. Again, consult your vehicle's specifications.
If you're not comfortable performing these tests yourself, it's best to take the car to a qualified mechanic for diagnosis and repair. A professional scan tool can provide more detailed information than a basic OBD-I reader.
