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'92, 2.5 engine, 5 speed manual, ~220k, weird problem.

Sometimes--not always--after I start the truck, back out, drive half a block, stop at the stop sign, make my turn, get up to 25mph and punch the clutch to slow down (speed limit there is 20mph and the local deputy hates me), my engine will rev. Ear-tach says maybe 3k rpm? It sits there until I put it back in gear. It may be ten miles before the idle will drop to normal (800rpm?). Once, it took about 40 miles before the idle dropped (but I was on the interstate, so I'm not sure exactly when it returned to normal). When it finally reaches a normal idle, it is usually there for the day.

This doesn't happen every time I start it up in the morning. Anecdotally, it does seem to happen more when the temps are <70* F, or when it rains or is foggy (but those days are usually cooler, so who knows?). No stalls, no stutters. Runs down the road at highway speeds with no issues, just a really high idle. MPG seems to have dropped from an average of 25 mpg to 22 mpg.

No noticeable vac leaks. Discussed it with some friends, everyone is pointing to the IAC or TPS. I searched the forums here, and I've got a pretty good idea how to clean the IAC. Weather permitting, I'll check that this weekend, and do some more checking for a vac leak (but the what I've been seeing makes me think it has to be something else).

Questions:
1) Is there any way to check the TPS, short of swapping in a replacment?

2) If cleaning the IAC fixes the problem, should I run the old IAC or replace it with new?

3) Suggestions of anything else I should consider/check?

While reading tonight, I realized I haven't checked the coolant in more than a month--if I've got a leak, I suppose it might be a problem with the temp sensor reading. I'll check that out tomorrow before going to work.

Thanks,
Jim
 

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There is a certain way to set the idle speed. There are you tube vids, but I found them unsuccessful, after trying those. I went back to the Haynes manual and found that more sufficient. (I know I should have did that to begin with, BUT it's a book, seems so less technically advanced.)
Quoting from the Haynes manual.
Note, This adjustment should only be performed on a throttle body that has been changed. The engine should be at normal operating temperature, before making the adjustment.
1. Remove the air cleaner housing.
2. Plug any vacuum ports as required by VECI label.
3. With the IAC valve connected or plugged into the wiring harness, ground the diagnostic terminals of the ALDL connector located under the steering wheel. ALDL is your OBDI port under the steering wheel under the dash.
4. Turn on the ignition but do not start the engine.
5. Wait for 30 seconds, for the IAC valve pintle to extend and seat into the throttle body.
6. Disconnect the IAC valve electrical connector.
7. Remove the ground from the ALDL.
8. Start the engine.
9. Adjust the idle set point as needed higher or lower.
10. Turn the engine off.
11. Reconnect the Connector to the IAC valve.
12. unplug any vacuum lines you capped and reassemble the air cleaner.

Hope this helps. I have been through the same things with my 92. It has been a long haul over 20 years. Engine was blown when I got it, and complications with the machine shop hindered getting things working properly, years ago. So it was stuck on the back burner for many years. Now it is a trip back in time to get it exactly right. Things sit and do weird things after laying idle that long.
Coincidentally for craps and giggles check your throttle cable. I had two women with an Isuzu Trooper and Rodeo that both had GM engines. There was a TAR like substance coming out of the inside of the cable. Now I figured it out on the Trooper by way of going down interstate and the gas hung after having to push the pedal down going up hill. Upon getting to the top of the hill, I had a slight run away situation going back down the other side. I have experienced what you have in my s10 so I would say that gunky problem is unlikely, as my S10 didn't have any gunk. But after that incident with 2 different models having the same thing, well yeah, too strange to ignore...
 

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I agree the Haynes manual is far from technically advanced. A lot of times they're far from correct.
 

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I agree the Haynes manual is far from technically advanced. A lot of times they're far from correct.
On the other hand, when you see so much false information on the internet. As well as these same problems popping up all over the this form. Then after doing it by how Haynes manual does it and it works. Well just sometimes it has it right on the head.
I think many of times when Haynes is wrong, well it isn't so much wrong as misinterpreted by the end user, or not explained in such a great detail by Haynes. Case in point.
2. Plug any vacuum ports as required by VECI label.
This is very misleading in a way. On my s10 and on some others there is a vacuum line going from the intake to the valve cover that is plugged to the "Dummy PCV", as my truck has a fitting on the valve cover, but when you take it off there is no valve. It is more of a draft fitting to draft the valve train area.
When the book said to plug off any vacuum ports after removing the air cleaner housing, I really didn't give much thought to plugging off that port. (or removing the hose and plugging the hose end) But you end up with a slight miss when you hook everything back up, if you skip plugging that one, because it is giving a false reading during the relearn status of the ECM.
As for saying they are wrong, I cannot say that. Misinterpretation can be the error of any human in the world.
I will say I have a slight library of Haynes manuals for a variety of vehicles and have been way more helpful than even the internet in most cases. Just have to look up the correct section for information, which is a little more burdensome.
 
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