Okay, I guess the idle problem with Preludes is one of the more common problems. We're seeing alot of these lately. A semi-definitive answer for the FAQ:
Problem: The car has erratic RPM idling. It may even stall (die) on you when you stop putting in the gas. When parked, you may note that the idle jumps up and down. When you rev, the idle goes up, starts falling to 0RPM, and then stalls. So why?
There can be a billion things wrong with your car that can cause a jumpy idle.
First thing I'd check is the TPS (Throttle Position Sensor). Use a voltmeter to check the voltage of the sensor to make sure it's working properly. If it isn't your idle will be erratic. Use a manual to find the proper voltage... I think it is .5 volts closed and a little less than that at WOT (wide open throttle).
Now check your IAC (Idle Air Control) Valve. It's on the back of your Air Intake Manifold. Unplug it. Doing that should make your idle go up if the IAC is working as it should. If it doesn't, you've found your problem. Go to the dealer, grab a new one, install it, and go out and buy a Coke because you're done. If your idle does go up, turn the little bypass screw on the side of the throttle body and bring it down to about 500rpm. Once you plug in your IAC valve again, you should be hitting normal speeds again, 700-800rpm.
If this fix doesn't work, you've got yourself a headache ahead of you. Check everywhere (!!) for vacuum leaks. All your hoses and lines. You need to seal up the leak. Doing so should fix up your idle problem. No? Well read on.
Now is where we need to determine if your car is stock or not. Messing with fuel/air can cause rough idle. Bring back fuel pressure to stock levels (get a manual for that). Sometimes if you bore your throttle body it can cause problems. If you have a chipped ECU (or a bad one), it could also cause problems.
If you're still reading, well... take your car to the dealer or mechanic. Because I've ran out of ideas.
