What can cause abnormally high idle speeds.
The first thing to do is to see if there are any diagnostic trouble codes stored in the PCM. If there are this will give us a good starting point for troubleshooting.
Many parts chain store will read your codes for free, all you have to do is ask. When you find these codes, you will be able then to follow them to possible causes.
Lacking any codes, I would have to start with the Idle Air Control Valve/Bypass Air Control (IACV/BAC). You can try cleaning it and see if that improves your idle speed. A throttle body cleaning is likely to cure the high idle speed as well.
Possible Causes of High Idle Speed
There are many possibilities when your engine is idling too fast. Here are some common ones to help guide you towards the root of the problem.
More: Car Idling Speed Problems:
- In vehicles that have carburetors, a bad accelerator pump or power circuit.
- The engine is overheating, and the cooling system needs repair.
- Fuel pressure regulator may be operating at too low of a pressure and would need to be replaced.
- Ignition timing needs adjusting.
- Ignition problems stemming from the distributor cap, rotor, ignition wires or spark plugs.
- The computerized engine control system is faulty.
- Vacuum leak
- Bad idle speed control unit
- Bad alternator
For the do-it-yourself mechanic, this is a problem you may be able to trace to its source with some patient troubleshooting. Some of the solutions are best performed by a service shop.