90 Lumina help

I was reading on another post here that has the same exact problem I am having. Here it is....

I have the wierdest problem with my lumina. I have a check engine light and when it's on.. the engine has a high idle and the speedometer doesn't work.. then.. sometimes.. when I drive it the check engine light goes off.. the idle goes back to normal and the speedometer works!

Any idea what it could be?

I checked the codes and the only one that came up was code 35 which is IAC.... Is that the only problem or will I have to replace the ECU like the previous post?

I'd try replacing the IAC first, and see if it fixes the problem. An IAC would be cheaper than a ECM. Of course you could just go to a yard and find an ECM from a 1990 Grand Prix, Cutlass, Lumina, or Regal with the SAME engine as your car.

I started off replacing the oxygen sensor, because it was pretty bad. Next I took a look at the VSS there was a bit of power steering fluid in it, which I got cleaned up but other than that it seemed alright. Then I replaced the IAC. This caused the engine to idle and run at a high rpm. I took the car to a mechanic friend of mine who hook up his diagnostics to it. He said that it took him a while to get into the computer, but when he was able to he reset? something? Anyways the car still was at a high rpm for a little bit after starting it up but then returned to normal. The mechanic thought that the computer was probably about ready to go. SO to make a long story short I replaced the ECM and now it works fine (knock on wood).

Speedometer is fine now too.......

He must have reset the idle speed. You can do that yourself by performing an idle re-learn procedure on your car.

That model Lumina (I know, I have one) is notorious for electrical problems when it gets up in years. I'm also surprised the ECM was the culprit. Although a bad ECM could definitely give you the problem you were having, and lots and LOTS of mechanics LOVE to recommend replacing the ECM, you are the first person I have ever heard of who successfuly fixed an electrical problem on a Chevy 2.8/3.1 V6 by replacing the ECM. Simply put, the electrical system on that model is crazy. I once had a battery that shorted one of its cells and it caused my check-engine light to come on and my speedometer to read 70mph until the engine sputtered and died.