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1987 Ford Ranger - 4.0 OHV - Revs High on Startup


Ubernater202

New Member
Joined
Mar 31, 2020
Messages
2
City
Georgia
Vehicle Year
1987
Transmission
Manual
Hello, I have a 1987 Ford Ranger 4x4 which originally came with a 2.9L. We did a 4.0 OHV swap out of a 91 Explorer. The issue I’m having I have not seen on this forum (either I overlooked it or it’s not there I’m not sure) but when you first start the truck it revs to about 3000-3500 rpms and then comes down to 1500 rpm it never goes below that. I’ve checked everything I know to check. MAF is good ICM is good. All my connections are good. The throttle cable isn’t getting stuck or anything and I just can’t seem to figure out the problem with this truck. I’m still young and inexperienced in the diagnostic side of things so there are likely many things that I am overlooking so sorry in advance. I think it’s a vacuum leak but I’m not sure how since everything is plugged and/or new lines. I normally wouldn’t really have a problem with this but that many revs on startup just can’t be good for the engine in terms of longevity! Any help or advise would be helpful, thank you!
 
Last edited:
I'd be inclined to agree with a vacuum leak. With the engine warmed up unplug the IAC valve and see what it does. If it still idles high or normal its most likely a vacuum leak.

Hoses are most common but intake gaskets can leak as well. Do a smoke test. Lots of DIY smoke test videos on youtube.
 
So with the IAC valve disconnected the truck should want to die? And I’ll look up some smoke test videos, thank you for your response.
 

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