2350 fuel rail decay

[Jokerone33]

So I have a 2017 Kenworth with a 2350 that’s been de mandated it has just over 229k on it the decay is so bad that when you shut it off while doing the fuel pressure test and turn it back on it’s gone from 29000 to a few hundred with in a few seconds the hi pressure relief valve is new and I had my shop guy tested it and said it works perfectly now when you’re running the pressure test you can hear the truck start running a lil on the rough side almost like it’s wanting to skip and if I run the cut out test it will run on one cylinder on all six any thoughts what it may be or what I should test next thanks in advance