* Throttle Position Sensor (TPS): A faulty TPS can send incorrect information to the engine control module (ECM), leading to a high idle. The ECM might think the throttle is open more than it is.
* Idle Air Control (IAC) Valve: The IAC valve regulates airflow at idle. If it's stuck open or malfunctioning, it will allow too much air into the engine, causing a high idle. Dirt and carbon buildup are common problems.
* Vacuum Leaks: A significant vacuum leak anywhere in the intake system will cause a lean condition, leading to the engine trying to compensate by raising the idle speed. Check all vacuum lines, hoses, and connections for cracks, holes, or loose fittings. Pay close attention to the PCV valve and brake booster lines.
* Mass Airflow Sensor (MAF) Sensor (if equipped): While less likely on a '94 351 (depending on the specific configuration), a faulty MAF sensor can provide incorrect air/fuel mixture data leading to a high idle.
* ECM Issues: Although less common, a problem with the engine control module itself could be responsible. This is usually diagnosed last after checking all other components.
* Throttle Cable (if equipped): On some older systems, a sticking or improperly adjusted throttle cable could cause a high idle. Make sure the cable moves freely and is properly seated.
Troubleshooting Steps:
1. Visual Inspection: Start with a thorough visual inspection of all vacuum lines, the throttle body, and the IAC valve. Look for obvious cracks, leaks, or debris.
2. Clean the Throttle Body and IAC Valve: Spray throttle body cleaner on the throttle body and IAC valve (following instructions carefully). This often resolves issues caused by carbon buildup.
3. Check Vacuum Lines: Carefully inspect all vacuum lines for cracks, holes, or loose connections. Use a vacuum gauge to check for vacuum leaks if possible.
4. Test the TPS: You can test the TPS with a multimeter to check its voltage output at various throttle positions. This requires a specific procedure found in a repair manual or online for your specific engine.
5. Check the IAC Valve (if accessible): Some IAC valves can be tested for proper operation using a multimeter, following instructions specific to your vehicle.
6. Check MAF Sensor (if applicable): If equipped, test or replace the MAF sensor if suspected.
Important Considerations:
* Repair Manual: A factory service manual for your specific 1994 Ford 351 is invaluable. It will provide detailed diagnostic procedures and specifications.
* Professional Help: If you're not comfortable working on your engine, it's best to take it to a qualified mechanic. Incorrect diagnosis and repair could cause further damage.
Start with the easiest and most likely causes (vacuum leaks, cleaning the throttle body and IAC valve) and work your way through the list. Remember to be methodical and document your findings.
