For months my truck has been running good except when I'm driving at freeway speeds trying to hold a speed with rpm around 1800 to 2100 the motor would surge or buck. Until yesterday I've been able to apply accelerator and power though it into passing gear and it would clear out with black smoke from tail pipe and smooth out and run fine again. Yesterday same thing but this time instead of it smoothing out it let out a loud blow off sound big cloud of black smoke and now I have no boost and a noise under hood every time I give it gas when the turbo should be spooling up. I think the turbo is bad but just don't know enough about diesel to say. That's why I need help from you guys. Don't want to spend the money if the turbo is not the problem. Please help!!