1987 Ford Ranger - 4.0 OHV - Revs High on Startup

[Ubernater202]

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!

 

[Dirtman]

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.

 

[Ubernater202]

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.