I have been helping a friend with his LLY duramax in a 2004 5500 gmc topkick. The problem we are having is that the diagnosis per the diagnostic tool is low rail pressure. but here are the kickers to this ,the engine runs great without any load , but under a load it runs normal up to around 55 mph then kicks into limp mode and falls on its face
1) replaced #2 and 8 injectors after balance test showed that they were bad
2) repaired injector wiring harness on #2
3) repaired fuel line from tank to ficm
4) replaced fuel filter
5) replaced fuel rail bypass valve with blockoff unit
6) ran check on scan tool again still same problem
7) Replace fuel injection pump with another that came off of a topkick that was upgraded to a high performance pump , still no improvement same conditions
8) replaced map sensor
9) NEED LOTS OF HELP PLEASE AT A LOSS
I am wondering if the problem is or could be any of these listed items and if it is how to test them to be sure FPR, MAF, or ECM and or FICM