Pontiac Repair: hard start & high rpm when starting hot, water corrosion, inner fender

Question
I have a 95 Grand Prix 3.1. Starts great when cold, once hot and you turn car off and restart, it does not want to fire right up and rpm will go up to 2500-3000 rpm and then come down to normal. On a few occasions it has stalled when stopped at a light. No codes in computer. Did put a new IAC and TPS on car, still the same. Had computer tested, checks ok. On a few occasions temp gauge would go full range, checked temp with infrared temp gun, seems ok.  Any Thoughts? Thanks for any help you can offer!  

Answer
Hi ron:

Do you have access to a Scan tool to read engine data? Not a code reader. If so there are a few things to look at when the engine is hot the IAC counts should be somewhere around 20 counts hot with no a/c on. More then that the throttle plate is dirty less then that I would start looking for a vacuum leak or a aftermarket PCV valve. Take a look at the MAF sensor reading at Idle They sould be in the 4-6 GPS range hot with no a/c on. what are the fule trim rates when it's running shouldn't be more or then 10% and no less the -10%. When the engine is running at idle your Idle % should he 0% anything more then that the idle speed will be eleivated. Do you live in the rust belt area of the country? Is so I would take a look at the engine contorl module connectors to see if an water/corrosion is present. The engine control module is on the right inner fender well under the aux. fuse box. Does it act EXACTLY the same now after the parts were replaced? did you use Aftermarket parts.. NEVER use aftermarket electrical parts on a fuel injected engine they cause more trouble then they fix. has any trouble codes set? have you checked fuel pump pressure at hot start and cold start. You should have pretty much insttant pressure at key on in the area of 40-44 PSI. it should not bleed off quickly with the key off it should hold for say 10-15 minutes before it starts to bleed off slowly.