So my car is idleing around 2k. The mechanic I've been working with says he changed the O2 sensors and the IAC sensors and has checked the vacuum hoses but the high idle still exists.

He's thinking it the CPU now.

I read somewhere that the CPU could be getting a signal that the engine is cold and therefore send more fuel to the injectors, which when I look in there, it does appear to be sending a lot fuel for just sitting there at idle.

My engine water temp is not hooked up to the dash at the moment. The wire needs to be replace and in the meantime we have installed an external water temp gauge.

Being that the dash water temp is not hooked up, is this the same signal that should go to the CPU and tell the engine to stop racing? Or is this controlled by a different sensor?

 

fix what is broke it will usually cure a problem

 

What is the history on the car? Is it stock? I chased a high idle for a loooonnngggg time becuase I was afraid to mess with the set screw on the throttle body. Turns out that that was exactly what I needed to do. After insuring there was no vacuum leaks and changing out the IAC, I tried pushing on the throttle plates to close them and sure enough, the idle dropped. The PO must have adjusted it incorrectly when putting in a 52mm TB. I backed off the screw until it wasn't touching the throttle bracket any more and then turned it back in until it was just putting pressure on the bracket (used a peice of paper as a gauge). The car idles perfectly now and the ECM can use the IAC to control the idle as it is supposed to. Before my IAC counts were at 0 which means it is maxed out, now they are around 20-25 IIRC which I believe is where you want to be.

Not sure what year you have, but on the LT1, the ECM uses the ECT sensor on the water pump. The other sensor is just for the gauge. If your TB or other source is letting too much air in, the ECM will know that and add in more fuel. The car will run rich in warm up until it goes to closed loop operation, but just dumping more fuel in wont cause a high idle without more air, it actually tends to lower the idle I think if it is just adding too much fuel.

So assuming that you have no vacuum leaks and your IAC/ECM is working right I would look at the throttle plates and see if they are stuck open or try to push on the top of the throttle plates as if you are trying to close them. If that changes things, I would suspect that someone has messed with that screw, especially if your car is not stock and certainly if the little tamper proof cap that is supposed to prevent you from messing with that set screw is missing. That is the problem with 20+ yr old sports cars, hard to find one that someone hasnt messed with at some point.

 

An idle that high is probably a vacuum leak.

What year is your car? Makes a LOT of difference. I would guess 84 based on your sig, but please verify that.

The ECM uses the coolant temperature sensor on the front of the intake manifold. Completely separate from the dash temperature sensor.

 

In my case the cables for transmission/throttle where adjusted wrong. It idled high and did not shift well. I had it adjusted and everything works well now.