You need a new idle control solenoid in the front of the throttle valve opening. While you're at it, remove and clean the idle air control valve which is in the same location.

I have a 99 Grand Prix with the above new parts that solved the problem. Buy them from a Chevrolet Dealer.

A 1998 GP does not have a carburetor it is fuel injection. I agree with Trevor either that or maybe fuel pressure regulator.

"RMP"? Do you mean RPMs? Sounds like a vacuum leak.

Check out the idle control valve . Can be partly clogged with dirt.

Explain in more detail please.

Ok so I have a 1998 Grand Prix and its been working great for the last year I've had it but now its acting up. When I start the car the rmp shoots up to 4 and it will go back down to three but no farther. Then when I tried to drive the car it shot right back to 4. My car has always been a little loud but that was never had a effect on the car. Could this be a problem with the carburetor? If anyone could help that would be great. ( I am taking the car to a mechanic tomorrow but I just wanted some insight before I went)