this kind of thing is really easy to diagnose.
there's only a few possibilities the most common being a restriction in the fuel system. It is easy to see it if the tech doing the work has a dyno to load the engine while watching the data stream (scv current rail pressure etc)
dealer I worked for didn't have no dyno which made it very very hard to do a loaded test on anything. With these common rail electronically injected engines a dyno is a necessity not just another tool. So what I usually had to do was isolate which part of the fuel system was at fault by removing the outlet line from the electric pump and putting the line into a jar or bucket, then turn the key on. Low flow is obvious at that point. If flow didn't drop off, it was going to be an issue beyond the lift pump, filter, pinched hose, rail, injector(s), scv, supply pump, wiring, ecu, etc. That's where the data needs to be available--once a knowledgeable tech sees the data, the problem is usually obvious at that point. Without the data (and the tools to read it) diagnosing a common rail engine is virtually impossible outside of just throwing parts at it or checking the obvious. I have seen both and it can real expensive if you don't know what your'e doing.
that said, diesel fuel (particularly off road) is dirty. And it's wet. it sits in the tanks underground where it gets moisture in it. Moisture and dirt. The filter is supposed to catch it but it don't get all of it, particularly moisture--and that moisture will destroy supply pumps and injectors. On a standard L (33 or 3901) just replacing injectors and supply pump is over 5 grand unless the price of the stuff came down which is unlikely.
but it doesn't matter because nobody wants to go see a dealer nowadays, they call them "stealerships"--that is, until the dealer is needed for something then it's OMG I shouldn't have said that I'm sorry I'm begging you to fix my stuff like yesterday and I'll pay $1 extra if you fix it before it broke.
