fast idle

I have a 1988 Thunderbird Sport with the 302 with SEFI. Has only 50,000 mi. on it. All of a sudden it picked up about 500 RPM at idle. I checked (visually) for obvious vacuum leaks and found none. I disconnected the throttle linkage and that didn't do it either. Any ideas? Which other components control the idle speed?
Thanks

Spray carb cleaner on all of the vacuum hoses and fittings where an air leak can occur, with the car running. If the idle speed changes you know that's where the leak is.

Thanks everybody, I found the problem. The gasket on the EGR valve was blown out. Have no idea why, but it was. New gasket, and all is well.
Thanks again!