Dodge Repair: Dodge Diesel loss of power, fuel delivery system, dodge 2500 cummins

Question
I have a 2000 Dodge 2500 cummins diesel that has a loss of power. I have checked for codes and codes are (P done) are all that are showing. So far I have removed the muffler and installed a straight pipe,changed the fuel filter, twice, shows no signs of trash or water, checked the air cleaner, changed the fuel lift pump, replaced the wastegate actuator, and just finished replacement of the vp 44 fuel pump and adjusting the volts on the apps.the truck does not seem to be getting enough fuel. There are no visible signs of smoke out of the exhaust.The truck has 150,000 miles on it and still has no power. Could you help with this loss of power problem?"

Answer
I cannot help you with your question for two reasons. First, my repair knowledge of diesel engines is somewhat limited although I can repair them when I have them in the shop. Second, it would appear that the trucks fuel delivery system has been modified, APPS voltage adjustment and a vp 44 fuel pump?, further limiting my ability to accurately answer your question.