I need to change fuel filters on my Mk25 in the next few days (just doing long term maintenance, shes working well but no idea how old these filters are, they are as she came from .gov). Am doing both filters fine filter on engine + coarser one/ water separator). I assume ill lose prime* when i do it. Ill make some mental notes on what it takes to restart it, write it up sometime.
Personally im planning on just using hand pump after filters are changed. The one on the engine, i may leave just lightly hand tight... and after some hand pumping, crack it loose, let air escape, retighten lightly, hand pump, break loose and let air out, repeat process until its just fuel coming out (i suspect it would be easy to get big air pocket in system, "burping" it like that might get it to where its mostly fuel, this has been my experience w other diesel engines, that you need way to get air out... which sometimes is essential, sometimes the engine can crank through the air, get running, bleed itself). Might also just leave engine filter barely cracked loose, pump until fuel comes out there.
Im not sure if the pump is upstream or downstream of the engine mounted filter in respect to fuel flow to the engine. Its something that i could figure out, just dont have it in front of me right now. But how its positioned could determine whether having filter slightly loose would suck air in or let air push out. I think its the latter.
If i was in a real hurry, id probably prefill the fuel filters (VERY CLEAN FUEL ONLY), install, pump until decently hard, then start engine let idle a bit...
ANYWAYS! Ill be doing this soon, ill take mental notes on what worked, and ill share them here
Ill follow through on that. Just be good information for us to have available. Trying to prime diesel engines can, as a whole, be a frustrating and variable process from one model of truck to the next. Some needing very particular steps, others not caring much...some in between
Notes -
By losing prime in this case (swapping fuel filters), the fuel system will prob be dry from engine mounted filter back to the tank, but there should still be fuel in a lot of the engine. Fuel pump to injectors should still be full. This is not a "complete loss of prime" like if you ran engine out of fuel or took a bunch of fuel system components apart and let them drain... and who knows what condition the OP's truck was in. Not sure if there are anti drainback valves in the fuel system / where, but wouldnt surprise me. That could influence some of these things...
Above when i just say "pump" im referring to hand pump
