Idles at 2500

[thedave]

I just picked up a 91 ranger 2wd with the 4.0 and auto trans. When I initially start it it idles right where it is supposed to, but after 10-15 minutes of running at 65 plus, when I put it into neutral or part it will idle at 2500 and not drop down. When I shut it off and restart it it idles fine again.

I thought it was the IAC as when the IAC is unplugged it drops down to the right idle speed and stays there, but I jsut changed it out with a new one and it is doing the same thing.

Any suggestions?

 

[Nhaz]

Yes as a matter of fact I have a idea. I experienced almost the exact same problem.

When it rains does the passenger side floor get damp? If it does. Or you haven't had it long enough to find out if it does. Pull the cover off the eec-iv computer on the passenger side.

You are looking for corrosion on that computer or inside it. When I finally tracked it down on my truck water had been pouring on top of it for many years prior to my owning the truck. And apparently its a very common spot for the truck to spring a leak on that side.

Mine was so bad the aluminum oxide powder from the corrosion was a inch deep right inside the eec-iv case. Causing many shorts. One of which is had the exact same symptoms of your idle air control not working correctly.

In any case I didn't attempt to repair that eec-iv, I just replaced it from a donor truck.

 

[thedave]

I haven't had it long enough for it to rain, but I did do some checking of some stuff and may have found the issue. I disconnected the TPS sensor and the idle went back to normal, reconnected it and it went back up to 2 grand.

I did some fiddling with it and there is just a little bit of adjustment in the screws, I was able to rotate it about 1/16 f an inch and that returned the idle to where it should be.

It seems to have done the trick so far but I am keeping an eye on it, and i'll check the computer for corrosion too next time I get a chance. Thanks for the tip Nhaz.