Im working on a 1992 3500 gm with a 454 bbc. My problem is it will not idle down after warm up, it stays at a constant 1100rpm. I have checked for vacum leaks, another shop has replaced the egr valve, I cleaned the IAC motor, checked the timing, set the tps, and even checked the throttle linkage. Also the coolant temp sensor appears to be in range. There are no trouble codes, I'm startint to get a little discouraged, there isn't muck else that pops to mind that I can thinmk of. it is a fire rescue unit, but as far as engine work done to it, I don't think there is any beyond pcm calibration. If any one has any good Ideas please feel free to let me in on them, any help is appreciated.