Ford engine revs high and stays there during shifts

[Roland]

1990 f250 4x4, 5.0, 5 speed, revs to 2500 and stays during shifting. I have to turn engine off to stop it. On cranking, engine revs to about 1500 for 10-15 seconds, then drops to about 650 and will idle all day at that speed. Top mechanic at national name tire and auto service center ( a friend ) replaced, in this order, throttle position sensor, EGR valve and sensor, idle air control valve. I replaced MAP sensor and oxygen sensor which made a big difference but did not solve complete problem. I also replaced a leaking vacuum hose on the throttle body and checked all other hoses. Now problem returns at times and I still shut off engine to stop it ....ANY HELP OUT THERE???? THANKS.....

 

[Kennyray]

I used to have an 88 that did the same thing. I had to replace the gasket under the upper plenum.

 

[Roland]

Thanks Kenny Ray for the reply about the plenum gasket. I forgot to mention that I had also replaced it using dowel studs to prevent slippage/distortion. My problem appears to be a vacuum leak that I am still searching for. The truck only has 105k miles on it and sat up due to original owners health. Its primary use now is for beach driving and surf fishing and this revving problem does not (so far) happen at slow speeds. If I don't clear problem soon it will go to the big Ford store for repair (OUCH) because I will need it for towing a boat.

 

[Mixedbreed]

Use wd40 to find it. Spray till idle changes then look for the leak.

 

[Black02Silverado]

Use wd40 to find it. Spray till idle changes then look for the leak.

I also have used engine starting fluid to check for leaks. Best thing with that is, it doesn't leave any residue.

 

[Mixedbreed]

That works just as well. I don't use it as its is very flammable and not friendly to a hot engine.