I purchased a 1990 iroc with the 5.7l 350 TPI auto transmission. The car idles rough but it is very random. Sometimes i can start it and it runs fine while other times it will jump from 1100 to 1500 on its own. It does the same thing when i rev it up for instance it will jump from 3000 to 3500. I have changed the intake manifold gasket, exhaust gasket, spark plugs (not wires), and the previous owner changed the distributor cap and fuel filter. The vehicle has 230,000 kms. Another issue not sure if they are related or not but the car seems to reach redline extremely fast and is doesnt seem to have much power. My guess is an EGR problem but i was hoping someone on here has ran into something like this or knows what i could try next. The vats system has been bypassed. Sorry if this is in the wrong place first time using the forum.

 

The normal things I would check is to make sure the EGR valve is sealing and clean the ICA and re-home it.

Search youtube for how to videos. Keep in mind your RPMs can NOT surge without an air supply. So the next question is the ECM telling the IAC to open or is something else going on.

 

So i made sure the EGR valve is sealing and we cleaned the ICA but did not adjust it. Any other ideas?

 

Like I said, is the IAC motor being told to change and reacting too slowly (AKA lazy IAC) or is there another source of air that is varying (like a brake booster that leaks sometimes). I would have no idea how to move forward without a scan tool.

Maybe eliminate all sources of possible air leaks? Even is your 100% sure its not a vacuum leak then you are still not sure if its the IAC being told to do something by the ECM or if the ECM is over reacting in each direction because the IAC motor is moving too slowly.