1991 with odd idle issue

[KMW-NWXJ]

I am looking at a really nice '91 my neighbor is selling. The only issue it has is a strange high idle. It is a 5 speed.

It idles at 1000-1200 until you come to a dead stop. As soon as it stops moving it drops right down to 800. You can coast to a stop in neutral or with the clutch in and it will stay idling high, even at like two mph. But as soon as you stop rolling it drops.

He has replaced all the sensors and cleaned the maf, and no change. It starts great, runs and drives awsome, just this odd idle thing. Any ideas suggestions? I searched around here and couldn't find anything quite like this.

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[nickyg]

Dirty Idle air control valve is the usual culprit. Followed by the throttle position sensor. then a Vacuum leak. I had a funky idle situation once and it turned out to be a clogged pvc elbow.
By the way, I wouldn't trust a new Throttle position sensor from a discount auto parts store. I had one that went bad after a month of service and the replacement was bad right out of the box. They are easy enough to test with a volt meter and only takes a minute.

Here's a few vids to get you pointed in the right direction

https://www.youtube.com/watch?v=cHiA2j5fLqk.

https://www.youtube.com/watch?v=URNYaGXM_uQ

[4.3LXJ]

There is one other thing. Was the transmission output speed sensor replaced?

[KMW-NWXJ]

There is one other thing. Was the transmission output speed sensor replaced?

No, I'm pretty sure he hasn't tried that. I'll suggest it.

I'm thinking that it ought to be something simple, just a matter of finding it. Is there anything major that would do this? So far it isn't worrying me much, but he is intent on fixing it before he sells it.

[4.3LXJ]

I am thinking that this sensor is sending the wrong signals to the ECM and telling it that the tranny output shaft is spinning faster than it is. At faster speeds, when you let off the gas it does not go to idle the same way as a carbureted engine does

[KMW-NWXJ]

That makes sense! I just tried it on my way home in my work car. Put it in neutral (it's an auto) and it holds at 1800-2000 until about 15 mph and then drops to normal 800.

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[KMW-NWXJ]

One question: does that sensor also run the speedometer? Because it seems to read correctly.

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[4.3LXJ]

The transmission controls are separate from the speedo controls.

[KMW-NWXJ]

Ok, cool. Thanks!

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