2002 duramax dulley. i have black residue all over the side and rear of my truck where the exhaust is run out.the truck runs a rough idile.could it be injectors.or at worst a ring.like a gas engine it does not smoke if it was a ring.could it be running rich.help is needed so anyones two cents would be appreciated.THanks my truck is sick is there a Dr. in the house...http://www.dieselplace.com/forum/smileys/Cry.gif