* Throttle Position Sensor (TPS): A faulty TPS can send incorrect signals to the computer, causing it to increase the idle speed. A failing TPS is a frequent cause of 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 can lead to a high idle. It might be sticking open.
* Vacuum Leaks: Leaks in the vacuum system can disrupt the proper functioning of various engine components, including the idle control system. Check all vacuum lines for cracks, holes, or loose connections. Pay close attention to lines connected to the brake booster, and the PCV valve.
* Mass Air Flow (MAF) Sensor (if equipped): If your truck has a MAF sensor (less likely on a '91 but possible depending on the engine), a dirty or faulty sensor can cause incorrect air/fuel mixture calculations resulting in a high idle.
* Throttle Body: A dirty throttle body can restrict airflow, causing the computer to compensate by increasing the idle speed. Cleaning it is often a relatively easy fix.
* Faulty Computer (ECM): While less common, a malfunctioning engine control module can cause various problems, including a high idle. This is a more difficult diagnosis.
* Fast Idle Cam (if equipped): Some vehicles have a fast idle cam on the throttle linkage. This is usually a mechanism to provide higher idle when the engine is cold. If this cam is stuck or not retracting properly, it can cause a persistently high idle.
* Automatic Transmission Issues (if applicable): In some cases, problems within the automatic transmission can cause a high idle. This is less likely but possible.
Troubleshooting Steps:
1. Check for vacuum leaks: This is the easiest and often most fruitful first step. Listen carefully around the engine for hissing sounds while the engine is idling. Visually inspect all vacuum lines.
2. Clean the throttle body and IAC valve: This is a relatively simple procedure that can often resolve the problem. Use a throttle body cleaner and carefully clean the IAC valve (don't force anything).
3. Inspect the TPS: Check its voltage output with a multimeter. You'll need a wiring diagram and a multimeter to do this correctly. There are usually specifications for its voltage output at idle and various throttle positions.
4. Check the MAF sensor (if equipped): Clean the sensor carefully with a MAF sensor cleaner. Do not touch the sensor element itself.
5. Check the fast idle cam (if applicable): Make sure it is functioning correctly and retracting as it should.
Important Note: If you are not comfortable working on your vehicle's engine, it is best to take it to a qualified mechanic for diagnosis and repair. Working on the fuel system and engine components can be dangerous if not done properly.
