2004 Very High Idle

Got a 2004 Aveo last summer with 61k miles. Did the timing belt kit and tune up so it ran fine. Then it started randomly idling at 2200 rpm. Bought a new Idle Air Control (IAC) unit and it worked fine... for a few days, then the idle went to 3000. Disconnected the Throttle Position Sensor (TPS), it ran normal a couple days... then back to 3000. Decided it might be getting too much air from the EGR, so I disconnected the entire EGR (and plugged the ends) and it ran normal. So I put in a used EGR..., but it made no difference.

If I goose the throttle to 4000 then hold the throttle butterfly closed, the idle drops down to 1100 for a second, then bounces back to 3000. So where is it getting all that extra air? While driving in gears 3-5 under load (without touching the gas pedal), it will only pull 2200 rpm, but when I put in the clutch.... back to 3000. So I just can't drive it.

The problem does seem somewhat temperature related because after an hour or so it rarely drops down to 1200, which is still too high.

I am completely baffled by this thing.http://staticaf.com/vbulletin/images.../runaround.gif The only OBDII code is the ''engine rpm does not match throttle position". How can it get that much air behind the closed throttle butterfly with the electrical connections for the: IAC, TPS, and EGR all disconnected? HELP!

Okay, I found out *all* the extra air for the high idle is coming through the idle bypass circuit. The brand new idle control valve is doing something extremely sinister- it randomly decides what speed the idle should be, and it does this even when the 4 pin plug is disconnected!!! Grrrr.

So each time I disconnected a component (EGR, TPS, PCV) it would go to a normal idle for a few minutes, hours, even days.... then go back to a completely random high idle. (though it is a bit temperature related)


Therefore I plugged the idle hole in front of the throttle butterfly valve- thus taking the entire idle circuit completely out of the equation. Now it never idles above 800 rpm. So I can now drive the car, but not very well when it is still cold.

I took my analog meter to check the 4 wire inputs to the idle control valve. There were three things which are very strange:
1) none of the 4 wires is a ground
2) with the engine off, each of the 4 wires measure about 20k ohms
3) when the engine is warm, all 4 of the wires would "pulse" between 1v and 11v at a one second interval

So is this the normal input for the 4 wires? HELP!

When you see a four wire circuit that is a current driven loop. I have attached a circuit diagram to the idle control valve. You can see four wires and NOTICE the transformer circuit between the two pairs.

This is really clever bit. The transformer acts a feedback circuit for position. So two wires is control usually 4-20ma and the other two is feedback.

Attachment 9328

After doing some more reading, the valve could be dirty or just bad. With the engine cold you should be able to remove it, leave it hooked up, plug the hole to the throttle body, the valve should open up all the way.

Then again, the valve should close when hot keep the idle close after your engine warms up the valve should close. to get it back to a decent idle.

Finally resolved this issue by buying a used throttle body (which included the TPS and IAC) for $45.
Car-Part.com--Used Auto Parts Market

Found out both of the new IACs I installed were bad and had failed in the same manner as the original IAC.
What are the odds of that happening?
So I chased this problem for 6 months.... when the first component I replaced should have fixed it.

Lesson learned: the $12.50 Amazon/Made in China IAC was not much of a bargain.
Thanks for all the help.