Hi everyone. I just bought this 94 F-250, 4x4, 5 speed with the 351 engine, and it's got a weird problem. It just randomly revs up and "idles" with the engine RPM very high. This seems to happen most of the time when the engine is warmed up, at first you could shut the key off for a moment and it would go back to normal for awhile, but now it will go back to normal for a few seconds and then rev back up. It also intermittently has hard starts (5-8 seconds), it doesn't seem to matter whether the engine is warm or cold. It also seems to have a very slight miss at normal idle.

I was told to replace the MAP sensor and did, and that does seem to have gotten rid of the Check Engine light but it hasn't helped my other problems. I'm thinking idle air controller, but my experience is in diesels and I'm shooting in the dark here. Any help would be most appreciated.

 

Its possible IAC valve, but when it fails it typically is bogging real low in rpm (almost to stalling) and then up to 1500 rpm. But it can still be possible.

 

IAC or TPS could be the problem. I have never had to replace a defective MAP sensor in the 20 years I owned my Bronco. Maybe I just got a great one from Ford? What is the "very slight miss" from as in when was the last time a tune up (plugs, wires, cap and rotor, fuel filter) was done? Your best bet would be to retrieve any diagnostic codes before throwing parts at it hoping to find the problem.

 

Check vacuum lines and pull codes

 

I honestly don't know the service history on the truck, and I've only had it for about 10 days. I've been driving it a little bit just to root out any issues. The miss is very minor, and undetectable above idle. I'm going to try to find someone with an OBD I code reader.

 

Me personally, i think this way. Perform yiur routine maintenance. Then troubleshoot issues. Sometimes they clean themselves out. And if not, well your truck should be maintained regardless.

 

Oh I absolutely agree, I just haven't had the chance to fly in on it yet.

 

Equis 3145, Ford specific OBDI code reader. $25 & up online + ship or available at Advance or Autozone for about $37.

Cheapest, easiest reader if you can't use a wire & count flashes.

Best investment you can make if you keep this truck.

 

Thanks! Found one at an AutoZone the next town over for $30. I have performed KOEO and KOER tests with it and it generated no codes. It had one code in memory, code 212 "Loss of IDM input to PCM or SPOUT circuit grounded." I don't know what that means, but it's not showing as a current fault. The truck actually experienced the revving issue during the KOER test, but it showed nothing.

 

Another update, I can find no vacuum leaks on the truck. If you unplug the IAC while it's idling high it will return to a normal idle for a few moments, then begin to stumble and eventually die. After that I unplugged the TPS and started it. It did eventually idle up, but not as high, then it began to stumble and finally died.

 

The reving is part of the ECU doing a test.

The 212 means that you probably have a grey ICM instead of a black one that your 94 should have.

This is the most recent thread on the subject that goes into detail about it:

https://www.ford-trucks.com/forums/1...-question.html

 

Well, with no obvious vacuum leaks and no codes I pulled a Hail Mary and replaced the IAC and the TPS, and also cleaned the throttle body. The plenum is pretty nasty as was the throttle body, but I didn't have the time to pull it all down last night. Any thoughts for a next step?

 

i have used a seafoam spray. Its for intakes. Takes a few minutes but cleaned up nice. I got some response back in my quick accelerations.

 

I have some that I may try, but I doubt it will help my issue. The IAC and TPS made no difference. It still revs up shortly after reaching operating temperature.

 

It has to have air to speed up. If air is not going past your throttle or IAC then you have vacuum leaks.

Stuff a towel, etc in a plastic bag and block the intake when it is idling high. See if it slows or dies.

Does it speed up when in gear? Does it pull, is it making power?

Maybe someone messed with the throttle stop screw.