Have trouble getting my old 2001 Cummins work truck to run. Truck ran great but was leaking fuel from drain plug on side of the fuel filter housing, so i changed it out. Truck was hard to start, then would run, but would die if i put it in gear. Did this acouple of times, then bucked and saw big cloud of grey smoke come out of tail pipe. Now truck is in what seems to be limp mode( iknow these older trucks dont have them) but truck will not go over 2000 rpm. Theres no more smoke at all, just has no power ,Thought maybe i had air in the fuel lines so ive tried first with the lift pump valve , and then cracked the injector lines, still nothing has changed. Totally stumped.