Valve Lashitis
 

Of all of the gmc v-6 engines I have dealt with, only one had the valves set right. Rough running and low power often are from valves set to tight or burnt valves from to little lash. Also the intake valve seals are always bad. They are the same as the chevy small block. Change em and oil consumption usually drops off. My experience is that a lot of these engines that supposedly needed rebuilt are suffering from these problems. BC