Hello all, I am still trying to solve my high idle problem. 1992 6spd 45,000 miles,the car idles at 1400 rpm. I bought the car with the high idle condition. I am confident I do not have a vacuum leak. I used a smoke tester and found no leaks. when the car is running I pull each vacuum line off the manifold one by one plug the source to verify that there is no leak in all of the lines. I have also jumped the ALDL and I only get a code 12
As a band-aid fix I made a manually adjustable copy of a IAC valve. picture attached. I have tried two replacement IAC valves with no change. The odd thing that I noticed is the length that I need to extend the pintle on my manual valve is about a 1/4" longer than the real IAC valve extends. I removed the real IAC vavle from the car after a shutdown when the car was idling at 1400 and the pintle is about 1 1/4" extended. With my adjustable valve I have to extend the pintle about 1 3/8" to get the car to idle at 750 rpm when warmed up.

 

Did you try removing the power brake booster vacuum line and plugging?

Smoke machine is great testing tool
When using, did you remove oil cap and check for "smoke"?--
Thinking you have a vacuum leak, intake gaskets are hard to diagnosis when they leak into the lifter valley area --sometimes will require taking it apart

 

I did not remove the oil cap when smoke testing. So far since owning the car I have replaced a leaking water pump, I replaced the intake manifold gaskets, mostly due to oil weeping on the block to intake contact area. I was hoping that when I did the intake gaskets I would accidentally fix an unknown vacuum leak, but no luck there.
I have one by one checked each vacuum line going to the manifold, plugging the manifold at the source.

Sam

 

I would block off the vacuum lines going to the brake booster and the one for the PCV system. Start the car to test the idle. Assuming that it still idles high with these two lines plugged then I would start looking at the IAC.

So the first thing I would do is verify that the IAC is in fact moving the way it should. Unfortunately you can't just plug in an IAC and watch it move without being installed in a throttle body. This can damage the plunger. So I use a salvage yard throttle body with everything removed. I take an old IAC and extend it out most of the way by hand and install that in the throttle body that is on the engine. Do not plug the harness into it. Then take a good IAC and put that in a junkyard throttle body and plug the harness into that one. Key on the car. In the first second the IAC should move to a "home" position then it should move to the "parked" position. Home will either be all the way in or all the way out, I can't remember off the top of my head. "Parked" will be around 40% open. Key the car on and off several times. You should see the IAC do this each time. Then start the car and let it run. As the car gets closer to operating temp you should see the IAC close more and more. At operating temp it should be close to all the way closed. Rev the engine. When you rev the motor the IAC should open into the "hold" position.

I was trouble shooting an IAC not long ago that had a pinched wire in the harness. The IAC would intermittently stop moving. The ECU would call for 0% opening and it would be stuck all the way open causing very high idle. It almost pushed me through a stop light at one point.

Most of the time issues like this are either a vacuum leak or carbon buildup inside the throttle body that limit the IAC movement. But sometimes it can be a bad wire in the harness, faulty ECU stepper circuit or as I read about online the other day a cracked cylinder head intake runner.

 

Your advice is very sound and welcome. I know I have gone thru each vacuum source on the intake, but I will double check them. I have long been suspecting the IAC. I have tried two new IAC's with no luck. My thought was that the IAC was not doing what it was commanded to do. To check that I made a visual rig to see that the IAC was doing what was commanded. Picture attached. I do not see the behavior that you described. what I do see is when I have the manual IAC valve in place and the car running at a manually set idle of about 750, I have a good IAC valve in the test fixture connected to the harness, I pull a vacuum line on the manifold and the pintle extends as expected. reestablishing the vacuum connection will make the pintle retract. I am at a loss why I have a 1400 rpm idle with a good IAC vavle installed and connected to the harness.

 

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I like that jig you made. Very creative. I always used a junk throttle body but your way works too.

If you have ruled out vacuum leaks then you need a scan tool. You need to see what the ecu is commanding vs what the iac is doing. Once the engine is at operating temp you can rev the motor and the iac should open up to its hold position. Then will slowly close again once your foot is off the gas. If you're not seeing that then you might have wiring issues.