[ob_1jr]

If it is an injector problem, take it to a GM dealership. The injectors should be covered 7yr/200,000 miles. What does the oil look like? Does it have diesel in the oil? Sounds like the guy that is working on it doesn't know too much about the DMax. Maybe it has a turbo problem if it is blowing black smoke and doesn't have a programmer on it. I would think there would be some trouble codes that could be downloaded to see what is going on. Fuel filter would be a good place to start though. Without actually driving or seeing this truck, it will be hard to narrow it down. Personally, I would get a VIN history report on it from a dealer and see how many problems it has actually had. Also, I wouldn't even think of spending my hard earned money on a problem child truck which could turn into a money pit.