Causes for Intermittent High Idle?

[Austin]

My legacy will idle high about half the time. Normal Idle is ~750 rpm. Sometimes it'll idle as high as 3000 rpm, sometimes around 2500, sometimes around 2000, and sometimes down around 750.

The car acts as if the throttle plate is sticking open.

I've removed/cleaned the throttle body and inspected the throttle plate and throttle cables and return springs. There is no binding or sticky action - the throttle plate is not getting stuck in any position.

I've replumbed the entire intake post-MAF sensor. Every hose is new and well sealed. The idling problem started about a month after I replumbed the intake.

I've removed/cleaned the IAC motor. There was a slight amount of gunk in the valve body, but it now is clean and operates smoothly. I also cleaned the inside of the coil and the valve slug, and there is no binding between the two. I've also checked the IAC motor with a multimeter.

I've replaced the TPS sensor. I've cleaned the MAF sensor...

Because the throttle plate isn't sticking open and there are no intake leaks, the only thing I can think of that would/could be letting that much air into the manifold is a bad IAC motor. Since it's an intermittent fault/problem, I'll probably replace the IAC coil anyway.

Am I missing anything obvious?

Can one buy just the IAC coil, or will I need to get the whole IAC assembley?

[ciper]

Purge valve allows air to enter the intake, maybe try plugging the inlet on the throttle body and see if it helps? No code will be thrown since the valve is still active.

[vrg3]

People have had bad experiences with cleaning IAC valves... The general wisdom has been that if you clean it out it won't be able to function properly because you also get rid of some of the grease or whatever that it needs. So, that could be part of your problem.

If it's not, though, then I have two other theories: maybe your throttle stop screw has been tampered with, or your IAC valve has been adjusted.

The throttle stop screw is the screw that the throttle cam hits when the cable has no tension on it. It's set at the factory and is never meant to be adjusted again, since the computer is meant to control idle speed itself. Many mechanics don't realize this and decide to adjust it thinking they're adjusting the idle. Most throttle bodies, before they leave the factory, have a little stripe painted over the screw and the throttle body with white paint, so you should be able to adjust it back fairly close to how it should be if this has happened.

You may have noticed that the IAC valve has two screws on top and so can be adjusted too. This is also never meant to be adjusted, but can be. You just loosen the two screws and then the whole thing can rotate. Again, from the factory, they usually have a small white stripe over the screws to mark the correct location.

If either of these were tampered with, you could get the symptoms you're describing.

[ciper]

I had to readjust my idle by turning the top of the IAC. It worked really well too. After a complete tuneup and cleaning the idle was slightly on the high side (though within spec). I waited for the cooling fans to cycle once and I adjusted the top so that I was dead middle within spec. I then left the battery disconnected overnight and let the car idle for about 15 minutes the next morning. I then cycled the AC a few times, repeatedly pressed the brakes to change idle and turned on high beams with rear defrost and cigarette lighter.

Thar car relearned to idle and now its rock solid. It actually idles better than my daily driver wagon that I do the most work on.

[Legacy777]

When you cleaned out the throttle body, was there black goo around it?

You may want to check out this thread
http://www.bbs.legacycentral.org/viewto ... =black+goo

as well as this one too
http://forums.nasioc.com/forums/showthr ... =black+goo

I've got pics in the one on nasioc.

If you have the adjuster screw, screw that all the way in.

http://www.main.experiencetherave.com:8 ... P_1511.jpg
http://www.main.experiencetherave.com:8 ... P_1512.jpg
http://www.main.experiencetherave.com:8 ... P_1513.jpg

Other thought is that the IAC valve is acting up. I get weird idling with mine when the A/C is on. It will dip down and then pop back up.

I think I have a leak somewhere in the intake track. Hopefully early this fall I can pull the manifold off and do some cleaning and work on the car.

[ciper]

All the way in? Are you sure?

[Austin]

Thanks for all the info!

I wish the dang thing was obd2 so I could plug my laptop into it...

vrg3 - I cleaned out the IAC valve body, and I also sprayed a bit of lube in it (lithium). After I was done cleaning it, it still operates freely. I'm not ruling out the IAC, but I don't think that's it...

777 - there was the standard black sealing goo around the throttle plate. Some is gone, and, like I said, there is a slight amount of light coming through the throttle body with the butteryfly shut... But, if it'll still idle at 750 sometimes, then it can't be the throttle body. If too much air was bypassing the throttle plate itself, then it would always be like that.

I'm going to replace the coolant temp sensor. If you think about it, an intermittently bad (not sure open or short, but it doesn't really matter) coolant temp sensor would cause all these things. Surging while driving w/steady throttle, higher than normal idle (if the computer thinks the coolant temp is 10C instead of 80 or 90C, it'll give a pretty large fuel enrichment).

We'll see if the coolant temp sensor fixes it...

Austin