I have a '89 F250 4X4 with a 5.0 V8 and 5 speed manual transmission. The idle has been high recently (about 1,000rpm) even after it warms up. It also will jump to 3,000rpm all by it self when stopped.The check engine light blinks on and off at times and I notice the engine knocks. I have replaced the trottle position sensor but have not noticed any improvement. The engine has 75,000 and runs great most of the time. Any help would be appreciated.

Thanks Don

 

When you replaced the TPS, did you check the PCM voltage to the TPS? If not, then that's probably your problem. On my Lightning, the voltage(with the eng. off and the throttle closed) should be no more than .91V. This voltage reading is what 'tells' the PCM the throttle is closed and then it sees that timing and fuel pulses are correct--I think .

The other possibility is your IAC is dirty or worn. They are less than $50 and take 5 mins. to replace...............

Dan