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Discussion Starter · #1 ·
Was just hoping somone might have an idea what's going on with my car's idle problems (90 DX manual). It runs fine when the engine is cool or even when it's hot, provided I don't run the AC. After some highway driving with the AC on, I get the high idle problem when I turn off the AC, but it seems ok when the AC is on. The tach runs up about 2k and surges from about 900 to 2k. Something keeps telling the EACV to stay open, because I can clamp the hose shut or unplug it and the idle drops down to where it should be, but there's still a small amount of surging going on. It seems like the problem only happens after running the AC on a really hot day. If I don't run the AC, the problem never shows up. After the car sits for a little while, I can start it up and the problem is gone.

Things I've done: Cleaned the EACV, cleaned the Intake Air Temp sensor, tested the Air Temp Sensor (within Helms specs), tested the TW (water temp) sensor (within spec), checked for air in the cooling system (bleeder shows no signs of air), reset ECU, and cleaned the TB. Everything else seems to be working correctly. I'm at a loss and any ideas would be greatly appreciated.
 

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First thing I'd do is remove the intake snorkel and with the engine running, put your hand over the throttle body. That should instantaneously kill the engine. If it doesn't it means there's a vacuum leak somewhere. By keeping your hand there in such a way as to not block all the air going into the engine, you might be able to listen for a whistling pinpointing the leak. Following steps are general surging idle diagnostic procedures.

Three things I know and have seen to cause a surging idle:

1) air in the cooling system - This will cause inaccurate readings from the various sensors sending the ECU on a loop of endless surging because it's continuously trying to fix the idle.

2) bad Idle Air Control Valve - on the back of the IM and should have a blue w/yellow and a black w/yellow wire going to it. Cleaning the filter screen inside the AICV will help restore a more steady idle. To do so, you must remove the valve assembly from the throttle body and clean out with brake cleaner.

3) A leak in the throttle body or intake manifold gasket - with the engine running, carefully spray brake cleaner on the mating surface edge by the gaskets. If the engine bogs when you spray, there is a leak where you sprayed.

There is of course the possibility of leaking vacuum lines. If you don't hear or see any obvious leaks, unplug all the vacuum hoses one at a time. Each time after you unplug one, plug the exposed nipple at the manifold or throttle body with your finger. If the surging goes away you know it involves that line and or any component that line goes to. After you do this, put that hose back on and proceed to repeat that for each and every vacuum hose and it may uncover the problem. Another way to help isolate a pinhole leak in a vacuum line is to put your hand over the throttle body with the engine running. A leak will give itself away by the whistling it will make. Note* cover up only as much of the throttle body opening as needed to allow the engine to stay running yet force the leak to reveal itself.
 

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Discussion Starter · #4 ·
Nope, no CEL or codes. Even though the TW sensor is testing within spec, I suspect that it could still be the culprit now too. I believe it could be just enough out of wack to cause problems, but still not throw a code. I guess I'll replace it and see what happens. Couldn't hurt.
 

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I think so too. If unplugging the IACV solved it (as it did with mine) then it's either the the coolant temp sensor (as it was with mine), the IACV, or the ECU. Start with the cheapest (they are in that order).
 
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