My dad has an 04 chevy duramax. Latelely the truck will load up at around 70 on the freeway. It will spit black smoke for a few seconds then clear up. Any idea what could cause this. So far the only thing done to the truck is oil changes and it has no more then 25000 miles
Mine was doing something similar, also would idle very rough at times. Found out it was a stuck injector. Local dealer had it fixed in one day.
Good Luck.