Pace car

What's up guys and gals. I owna 1998 Pace Car and I am having a very serious problem with idle speed. Car starts fine but then revs up to roughly 1800 RPM while in park. While stopped at a light the car is at the same 1800 rpm. Once you put the car in park the RPM's increase to slightly over 2100rpms. The codes given are for the idle sensor and the throttle position sensor. I have since replaced these with no luck. Took car to a local mechanic who told me I had put the Throttle Position Sensor in wrong and he claims to have corrected the improperly installed sensor only to have the same thing happen. In order to get the rpms to where I can drive the car I backed off the idle screw that holds the gas pedal. Got the car to idle at 900 rpm but then the transmision would not shift into the other gears until somewhere around 4000 rpms. Re-did the screw and got the car back to 900 rpm with the transmission in park and the car now shifts fine while in drive. That lasted only a few starts. It is now back to high revs (1800-2100) while stopped and in park. What could this be? It is driving me nuts!! I am looking for a new mechanic but came here to see if anyone has ever had those same issues. Have never done anything else to the car except put in a new couple into the Supercharger and the car ran fine after that for about 2 months. I NEED HELP and I need it NOW!!! Thanks

Do you have access to a live data scanner? You need to check the TPS reading on a scanner to see what it is doing. Voltage may check fine at the TPS, however, it may not be the same on the scanner. If this is the case, you have a bad PCM!

Have any modifications to the throttle body? I'm unaware of a pace car GP. What motor is in it?

If I were you I would check for a vacuum leak first off. Next I would pull the vacuum line off the fuel pressure regulator, turn the key on and off a few times to prime the system, and check to see if gas isnt leaking from the FPR into the vacuum line. And finally I would unplug the MAF sensor (common problem on 3.8's I understand) and start the car with it unplugged to see how the idle is.

I'm unaware of a pace car GP. What motor is in it?

A box stock L67 SII 3800 just like the GTP. Pontiac made these limited production models which were primarily badging, paint schemes, and a few other options. There was a Daytona 500 Pace Car and NASCAR Pace Car. A few of our old forum members had them. Each was assigned a specific production number since it was limited production only.

Other '97 and up GTP model variations was the GTX.

I have no modifications to the throttle body. I have checked regular vacuum leaks like things unplugged and that sort but found nothing. I will try to get the vacuum checked the way you suggested. Thanks and I'll keep you posted.

Will check into the PCM. Sounds promising. Thanks

Before wasting money on a PCM module what about the idle air controller (IAC)? What have you done about the IAC DTC and what was it. Whenever you post a problem with a SES/CEL light or any DTCs from a scanner it is highly recommended to post any and all DTCs. :grinyes:

I recall working with another member with the same or similar issue and the PCM did not resolve his issue. I'll see if I can dig up the old thread for reference.

Even though IAC was replaced did you check the PCM 5 volt reference input signal and the output.