i have a 1997 chev 3500hd w/ 6.5 turbo it just started having this problem the trucks starts and run real good for the most part but when im driving and the rpms get to 2200-2600 usally right before the transmission shifts it starts to dump black smoke out the tail pipe and loses power if i let off the throttle and the truck shifts to the next gear the smoke goes away and is ok till i hit that rpm range again it is worse if the truck is loaded.
the truck has 120,000 mile on it
thanks for any help.

 

any codes. trace your vaccum lines going from the turbo to the vaccum pump.

 

I have the same ex thing going on did you find out what it was thanks Bo