I have a 1998 GMC C3500 2wd Chassis cab with the 5.7L Vortec and a 5spd manual. I was wondering if you guys can help me with a problem that I've been having.

My problem is that whenever I come to an intersection, stoplight, etc. I let off the throttle (hoping the engine would help slow me down), but whenever I let off the gas,drop to third,(fourth on highways) and let out on the clutch the engine revs up and then will stay at a speed that will let the truck freewheel (coast), it holds its speed and doesn't accelerate or slow down.

There are no codes. I was wondering though, do I have a bad sensor, the wrong PROM, or a bad PCM. Or do I just need to disconnect the battery to relearn my driving (I just bought this vehicle, could the previous drivers driving style cause this?)

Any help would be appreciated, thanks!

Does the tach hold at whatever rpm it jumps to on the downshift or just the speedometer? Do you notice an audible drop in engine rpms after you downshift, or does it just stay at the level it is when you let out the clutch? Any nasty smells like a burnt clutch? Only thing I can think of by your description is that the clutch may not have enough clamp load to hold at higher rpms letting the clutch slip which will prevent the engine from slowing the truck. Just a wild guess on my part.

PS. Go to the ZF forum and sign up for the Brotherhood of Handshakers.

Slipping Clutch?

Does truck have a hypertech in it? Why I'm asking is I had a 98 K1500 w/hypertech installed... after a few months it didn't want to decel when I let off gas pedal, I'd uninstall chip then reinstall and problem gone... for a few months

That said... put a scanner on it and drive it, watch IAC counts during decel, if counts drop but engine speed stays same then IAC is shot, if IAC counts stay same or rise then you have ECM or software glitch. After that watch TPS sensor voltage and throttle % reading on scanner, should roughly agree with each other. Throttle closed will be roughly .5volts and 0-10%, on up to almost 5volts and 100% at full throttle.

It's easy to raise idle rpm but almost impossible to drop it lower than what ECM wants, even artificially raising idle rpm will set IAC code.

First thing to check is the ECM and sensor grounds... I know I shoulda put this down first but the rum is starting to kick in.