Hello,
I have a 1994 Ford E-150 5.8 V8 EFI and it has a high idle almost like a rocket or plane running and it does it from start up and won’t stop I talked to some older mechanics that know more than I do and they didn’t know anything about it and wanted to argue that in 1994 they didn’t put that engine in the vans so they wouldn’t help either because of that so it I replaced the throttle body gasket, throttle position sensor, Idle air control valve and cleaned the throttle body thoroughly and it still hasn’t fixed it I have checked for vacuum leaks also and another mechanic showed me how to test the PCM to see if it was bad and it wasn’t so at this point I’m stuck as to what the issue could be and it was dealership mechanics that I know and asked and they didn’t know either. I bought a Ford OBD 1 scan Tool but it doesn’t have a check engine light so I can’t use it for this issue could something be wrong with the ECM possibly or is the throttle body itself bad? I don’t want to keep throwing parts on it if it’s not any of those. It has been sitting for about a year but has been started and it just started doing it a few months ago so I don’t know if that would be something to do with it or not.

 

Need some more info,
Are you sure it is a 5.8?
How long have you owned it?
Has the engine been replaced ?
What is the 8th digit of your vehicle's VIN ?
Normally idle RPM is controlled by the PCM based on inputs from sensors.
Usually a vacuum leak , could be intake manifold gaskets, will allow addl unmeasured air to get in and the o2 sensor will report this and the PCM will control fuel to bring to correct idle speed.. How is the PCV valve and hose?

 

If the engine is running way too fast when starting a cold engine, the O2 sensor can be rulled out since its information is used after a few minutes of run time. I would use carb cleaner to spray around intake gaskets and see if the engine is affected. Major vacuum leaks will make the engine run way too fast.You can hang a vacuum gauge off the manifold and see if you have a steady 18-22 inches of vacuum. In addition to the intake, look at areas where there are large sources of vacuum such as the lines feeding the brake booster and the PCV valve. Make sure the throttle cable and the throttle shaft in the throttle body are not mechanically binding. If you still suspect the ECM, take its cover off and look inside for any buldged out or leaking electrolytic capacitors. There were a lot of faulty capacitors which were not up to the job installed into many products including ECM's. Any buldged out caps is reason for concern. They can all be replaced or replace the entire ECM if you do not have the skillset.

 

Thanks for the reply the 8th digit is an H and on the door panel where the info is it says 5.8. We have owned it since 1999 second owners. Original engine. I’m not sure where the PCM is. I will recheck for vacuum leaks but I couldn’t find a vacuum line diagram so it was hard to do. I don’t know how the PCV valve is or where it is since everything is so compact in it.

 

I do know the linkage is not binding I did look at that. I will try the other stuff as mentioned and maybe be able to find something.

 

I did have a throttle body with throttle blades/ shaft which were not returning to their stop. The hardened steel shaft for the throttle had worn and scored the aluminum bushings in the throttle body.

 

Just out of curiosity, have you removed the doghouse so you can see the back of the engine?

 

Something you might try,
Remove the PVC Valve and hose from the valve cover and plug the opening so no air can get into the valve,Idle may drop some.
Remove the oil fill cap and shoot a good shot of Carb and Choke cleaner into the oil entrance and note if any change in engine speed,
If so that would confirm intake gasket(s) leaking internally where not visible from outside.

 

Yes I did take the dog house off and ok I will try that and test the intake gaskets