Idle Speed & Speedometer Read-out?!

[RemoteUzer]

Hello, I've looked around but couldn't find anyone with my exact problem. It started off as just the digital speedometer on my 1988 Cutlass Supreme 2.8 cutting out occasionally. After a while, I would let off the gas and shift into neutral, to coast to a stop (manual transmission here), and the engine would jump to 2-3k RPM. Now it's to the point where the speedo still cuts out (quite frequently) and anytime I take my foot of the gas, the engine dies--extremely weak idle. I've replaced the VSS (no problem with original), I've cleaned the TB thoroughly to try to fix the idle problem, I've checked the IAC valve and it reads OK, I've shorted the diagnostic port but it only reads a code 12 ("all systems okay"). Basically, it's got to be something after the ECM otherwise it would be yielding a code (ie-broken sensor, malfunctioning circuit, etc) but I can't track it down. My two guesses at this point are either the Cruise Control Module or the ECM itself. Any suggestions? Thanks for your help!

[RemoteUzer]

I figured out it was the ECM. GM part number 1227727. I went to a junk yard and got a newer one out of a 1990 Grand Prix. However, upon installing it, the car blinked error code "33" (MAP sensor voltage too high), the speedometer was way off (reading 40mph as I'm passing people on the highway) and the shift light wouldn't work. Obviously the ECM worked, but it wasn't programmed for my engine/car. So I opened the PROM doors on both ECMs, switched the old PROM into the Grand Prix's ECM, reinstalled it, and it runs great now! Apparently, just the ECM board was shot and not the PROM. Hope this is able to help someone else someday!