Bought a used blazer with a 5spd last year... never driven an SUV with a manual before; it's pretty neat. Anywhoo, I've noticed the engine behaving strangely when I let off the throttle in gear, and I can't seem to find anything in the tech manual about what's going on. Maybe someone here can shed some light on this issue.

Basically, engine braking is very limited for the first five to ten seconds after I let up on the gas in gear. It's as if the computer were giving the thing enough throttle to keep it going at constant speed. Then, after 5-10 seconds, I am abruptly jerked forward and it starts engine braking normally. The problem is especially noticable in second gear near 3000rpm. Needless to say this is extremely annoying to both myself and the people behind me in traffic.

To make things even weirder, the other day I upgraded my horns and obviously had to disconnect the battery. When I took it out for a test drive (honk honk! hehe), imagine my surprise when the engine braking issue had vanished completely! It was working exactly as expected, transitioning from acceleration to decelleration smoothly as I let off the gas in gear. It was also stalling out every time I engaged the clutch without giving it gas, but that's probably unrelated. Gradually it has returned to the previous annoying behavior, but I anticipate that if I disconnect the battery to reset the PCM it will start working properly again (for a while).

Any of you folks have a clue why it behaves this way? At first, I thought this was a "feature" that GM added for some crazy reason, but now that I see it goes away when the PCM is reset I'm back to scratching my head. I have replaced the engine oil, tranny fluid, and the fluid in both differentials recently, but I don't know what maintenance was done before I bought the thing.

Thanks in advance!

 

Does it have cruise? It could be something going screwy with the apply/purge solenoid for the cruise control. If it has cruise control, I would first try to disconnect the cruise cable from the throttle body. I kind of don't think this is the problem, but it's easy enough to rule out.

While you're at it, it's probably a good time to pull off the throttlebody and give it a good cleaning. The fact that it didn't do it after a PCM reset leads me away from a sticking throttle shaft, but it won't hurt anything to clean the TB.

I do suppose that this could be all related to a clogged or sticking IAC (idle air control) valve which is located on the back of the throttle body and should be cleaned when the rest of the TB is cleaned. Some good TB cleaner (sensor safe stuff) and an old toothbrush will make quick work of cleaning the TB. I went so far as to pull out the throttle plate and give the bushings inside the TB that the shaft rides on a good cleaning, but you'll need some loc-tite to put the screws back in. I also lubed the shaft with some synthetic grease I have. On the IAC valve, make sure that the pintle (sealing cone) is clean and free of major indentations. Clean the pintle mating surface inside the IAC bore. Also clean out the passage where it feeds into the main throttlebody bore.

It definitely sounds like something electrical.