Truck is a 2002 Duramax, have not had any problems with it to date but lately the truck has been belching black smoke under heavy acceleration. Doesn't matter if the terrain is flat, up or down hill and this is happening without the truck being loaded. This truck has zero performance mods, bone stock! My question is, is this normal or is there something wrong? The truck seeems to been running as best I can tell, mileage has dropped a little. I know that diesel engines smoke but I haven't seen this from very many Duramaxs.