This is a carryover from a previous thread that got really long. I have a1997 K3500 and just got the glow plugs fixed, so I can start it in weather under 60 degrees. As I stated before this problem is intermittent. Sometimes just after idling, and sometimes when I’m driving down the road. What happens is that I step on the accelerator to rev it up and when I let off it will pop and blow Black smoke until the RPMs drift down to about 1500 or less and then like a switch it immediately smoothes out and drop down to 900 rpm’s or so on the coast. If it does it when I’ve just had it idling in the driveway, it will do it in park or neutral but as soon as I shift it into gear the popping immediately stops. I hauled a trailer and old truck with it yesterday for about 50 miles and it did it the entire way, also puffing out black smoke during acceleration. Then I drove back without the trailer and it did not pop once. I put in a new air filter & opened the front T valve with no stalling out. I am stumped.