* Throttle Cable/Throttle Position Sensor (TPS): A sticking or damaged throttle cable could be preventing the throttle from closing fully even when you release the accelerator pedal. Similarly, a faulty TPS doesn't accurately tell the computer the throttle's position, leading to incorrect fuel delivery and high revs. This is a very common culprit in older cars.
* Vacuum Leak: A vacuum leak in the intake system can disrupt the signals to the throttle body and cause erratic engine behavior, including high idle and revving. The vacuum system is critical for many functions in a carburetor-equipped vehicle (though less so on fuel-injected cars), and leaks can create various unexpected outcomes.
* Idle Air Control (IAC) Valve: This valve controls the amount of air entering the engine at idle. A malfunctioning IAC valve can cause the engine to idle too high or rev up unexpectedly. This is more likely on fuel-injected engines.
* Transmission Issues (less likely): While less probable, a problem with the transmission's linkage or internal components could cause the engine to rev higher than expected, especially under braking. However, this would likely be accompanied by other transmission problems.
* Computer/ECU Problems: Though less frequent, a malfunctioning Engine Control Unit (ECU) can lead to erratic engine behavior, including the problem you describe. This is usually accompanied by other problems though and harder to diagnose.
What to do:
1. Safety First: Do not drive the car until the issue is resolved as it could be dangerous.
2. Check the Obvious: Inspect the throttle cable for any signs of damage, fraying, or sticking. Try gently working the cable by hand to see if it moves freely.
3. Listen for Unusual Noises: Listen carefully for any unusual hissing or sucking sounds that might indicate a vacuum leak.
4. Professional Diagnosis: This is likely beyond a simple DIY fix. A qualified mechanic should diagnose the issue using a scan tool to read codes from the car's computer. They can then test the TPS, IAC valve, and other components in the system to pinpoint the exact problem.
It's crucial to get this fixed promptly. Driving with the engine constantly revving is not only dangerous (increased wear and tear, overheating), but also inefficient and potentially damaging to your car.
