Greetings:

I am new to this forum, having just purchased a 1992 Explorer, 4.0 engine (I think), standard transmission, 2 wd. I live on Saipan, an island in a U.S. Territory in the Pacific, and there are not a lot of good mechanics around here, so I have to stumble through on my own generally. I subscribe to alldatadiy.com, and added the Explorer yesterday, so I have access to that manual. The current problem is a slow return to idle. Upon cold start, the engine revs up too high, to about 2,500 rpms (estimated - tach does not work)then quickly drops down to more a reasonable level (maybe 1,500 rpms) and then drops to a normal idle within a minute or two. After that any time you give it any throttle it takes its sweet time returning to idle. The throttle body has a tag on it warning not to clean it. It doesn't feel like a sticking accelerator cable, but I have not had time to check it thoroughly. Any suggestions would be appreciated.

 

Check the IAC valve. There is a good post on this at the top of this forum.

 

Hummmm. Could be the IAC - Idle air control valve. It can be dis-assembled and cleaned.

It might be a vacuum leak - and it really sounds like it from what you describe. Check all the vacuum lines for cracks and especially the vacuum tree at the back of the intake. I lost a plug there once and I got all your symptoms.

But also, it sounds like the motor is doing what it is supposed to, in general. When it starts cold, it revs high so the motor warms up fast - less emissions with a warm motor - then it drops down a couple of increments until it know's it is OK and wont stall.

Once warm and you rev it up, it should drop pretty quickly down to 650 RPM, but there are still those incremental steps it goes through. So it's hard to say if what you are seeing is normal or not without witnessing the event in person.

I'm curious too if you have gotten a check engine light - CEL? It would be good to pull any codes out of the computer. There are webs site that tell you how to do this with a volt/ohm meter or using the CEL - do a google search. Or if you have some access to a diagnostic code reader you can hook up to the car, this is easier. Might as well figure this code stuff out now because you will need to now how to do this sooner or later.

Lastly, once you check the codes, it would be good to disconnect the battery for 10 minutes to clear the computer's correction factors. By doing so, you can tell if the problem is there all the time or a learned condition. Too much to describe on this aspect here in one note but let us know what you find and then we can help direct you.

 

I removed and cleaned the IAC valve. No difference. I did note that if I disconnect the valve, all of the malfunctions disappear and I have normal operation, just no variable idle. Does this finger the IAC valve conclusively? This seemingly stupid question is brought about because the valve does seem to be working. Is it possible that it is getting some bad input which creates the slow return to idle and excessive cold start rpms, and the bad input would have the same effect on a new valve? I think the answer to that is probably not, but I am unsure. Would hate to put a new valve in there and have the same problem.

As far as the codes go, I have not checked them. I did disconnect the battery for an afternoon to let the car unlearn any bad habits it might have and could not detect any difference. The check engine light comes on for a few seconds and goes off after you turn the ignition on - normal operation.

 

The IAC input comes from the computer program in the PCM. The prgram tells it to open x% under different conditions. As I first mentioned, what you describes sounds totally normal for the cold start to wram up. After it's warm, you should see a fairly uniform rev up and rev down in nuetral or park. Now after cleaning it, if it still hangs up there when you rev it up after the engine is warm, the IAC could be sticking. Cleaning doesn't always fix it. But before replacing the IAC, again, check for vacumm leaks. Eliminate this first.

Now as far as disconnecting the IAC and getting a normal idle - well first off I would think the engine wouldn't idle on it's own with out giving it some throttle. But regardless of that, since the IAC is not opening at all with it disconnected, the PCM would have no control over the stepped idle speed reduction. So it would rev up and down like a carbureted car. This test you did does not prove the IAC is good or bad. But it might prove that you don't have a vacuum leak.