* Idle Air Control (IAC) Valve: This valve regulates air flow at idle. A dirty or malfunctioning IAC valve is a very common cause of erratic idling, especially one that improves after driving (heat might clean it temporarily). It's relatively inexpensive and easy to clean or replace.
* Throttle Position Sensor (TPS): The TPS tells the computer the throttle's position. A faulty TPS can cause incorrect fuel delivery and idle problems. A bad TPS can lead to inconsistent idle and poor drivability.
* Mass Air Flow (MAF) Sensor: This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor will throw off the air/fuel mixture, resulting in high idle and poor performance.
* Vacuum Leaks: While less likely given the intermittent nature and improvement after driving, a small vacuum leak *could* be causing the issue. These are notoriously difficult to find, but a visual inspection of all vacuum lines is a good first step.
* Transmission Issues (Less Likely): The "kick down" and sticking might indirectly relate to a transmission problem *if* the high idle is a symptom of the transmission struggling. However, transmission problems usually manifest more clearly as shifting issues than solely high idle.
* Computer (PCM) Issues (Least Likely): While a failing PCM is possible, it's less likely to cause an intermittent high idle that improves with driving. This would usually manifest as a range of more persistent problems.
Troubleshooting Steps:
1. Check for vacuum leaks: Visually inspect all vacuum lines and connections for cracks, breaks, or loose fittings. Use a vacuum gauge (if you have one) to measure vacuum at idle. Low vacuum suggests a leak somewhere.
2. Clean the IAC valve: This is the easiest and cheapest first step. Remove the IAC valve, clean it thoroughly with carburetor cleaner (following the cleaner's instructions carefully), and reinstall it.
3. Inspect and/or replace the TPS: A multimeter can be used to check the TPS voltage output against factory specifications (find these in a repair manual). Replacement is usually straightforward.
4. Clean the MAF sensor: Carefully clean the MAF sensor with a MAF sensor cleaner (never use anything else). Again, follow the cleaner's instructions precisely.
5. Check for error codes: Use an OBD-I scanner (appropriate for your 1991 Bronco) to see if any diagnostic trouble codes (DTCs) are stored in the computer's memory. This can give you valuable clues.
Important Note: Working on fuel injection systems can be dangerous. If you're not comfortable working on your own vehicle, take it to a qualified mechanic. Incorrectly diagnosing or repairing these components could damage your engine. A repair manual specific to your vehicle will be invaluable.
