I think I can monitor fuel pressure with my standalone banks monitor when driving just need to figure out how to change the parameters I currently have it monitor new fuel filter assembly is in I pre primed it with a mixture of marvel, power service and shh off road diesel. Orings didn't look to bad minus a little winter grit around them, lower valve was definitely on a little to tight for my liking anyway I'm hoping the lower drain valve is ok cuz I didn't replace that. Anyways left a clean white rag around filter, using off road diesel and marvel it's much easier to look for leaking. Hopefully reassembly of fender liner goes smoothly.You need to get a reading of what your fuel pressure is doing. Is it dropping off under load or is the regulator going bad?
I'm not real sure if Dmax have a fuel pressure sensor, but they are not hard to add. You need to see in real time what your fuel is doing. That could point to a lift pump or leak.
We have great technology today, use it.