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I just bought a '92 F250 XLT 4x4 5.8L. I don't know much about car mechanics at all but I'll describe the best I can. When the truck is in park and while it is driving the engine revs up on its own for a few seconds then will rev back down. Now, I just had the catalytic converter removed today because it was clogged (shhhhh!), I will pick the truck up in the morning. Is it possible that my issue was because the engine was struggling to get the exhaust out? I did some looking around online and found that it may also be a vacuum leak, a bad Idle Control Valve, or a bad EGR valve? What do you guys think?
My bet would be on a vacuum leak, check the small lines (either plastic or rubber) all over the engine, none should be disconnected. The IAC (Idle Air Control) could be another cause, but then it would only manifest when you are actually idling the engine or under very light loads. If all the vacuum lines seem ok then you should probably check for codes, if any sensor isn't right there should be some codes stored. Do a search on how to retrieve codes, it isn't too complicated, but if you don't want the hassle you can buy a Equus 3145 (OBD I Ford code reader) or similar to make things easier.
I picked up my truck this morning from my welder and with the clogged catalytic converter out of the picture the truck seems to be running fine! The idle is still a little rough (struggling a tiny bit at some times, I think it could be the fuel filter) but it is not revving up like it was before, and I'm not having any problems at all while driving. Now, if the high revving comes back would I be able to rule out a vacuum leak? I would think if it was a vacuum leak it would happen all the time and not just off and on. Would I then look at some of the sensors?