I posted on here in jan that i sold a truck to a friend. It was a 1994 K1500 6.5L TD S engine, 4l80 tranny, 4X4. Has 210000 miles. It started blowing smoke one day and no power up 2000 rpm. He parked it in the garage till he figured out what he was going to do with it. He tryed starting it after sitting for 2 weeks and it wouldn't start. ( it's a heated garage) Tryed it again a week later and it fired up but still ran the same. he said screw it and started driving it. About 2 weeks ago it started running great. Had power and no smoke of doggy ness. What the Censored . Been running good since. He did put in a new lift pump and was planning on putting in a new IP pump. Why did it start running right? Why did it start running bad?:help: I don't get what's going on. Is the pump or the pmd or both? Any thoughts would help. Thanks

Something as simple as bad fuel?

Or the LP was bad and it was sucking in some air along the fuel lines. Did he do a new fuel filter too? With new LP there is positive pressure and maybe a small leak thats not noticed.

Could have been some electrical gremlin too that the warm garage cured. Time will tell.

Bad fuel is also a definate maybe.

Plugged air filter causes black smoke, as well as failing vacuum pump and WG solenoid

i agree with the post from GMCTL - wastegate solenoid. You can get the part and replace it yourself.