HELP! TRUCK ONLY STARTS WITH STARTER FLUID

Yo! Guys I have the same problem, but I didn't replace the fuel/water separator, just the filter.
The truck cranks but doesn't start, only with starter (which I know it is bad for the engine and only done 3 times so far) Lots of black smoke once I'm on the road, I'm assuming the fuel isn't getting burned properly.
The problem started gradually, it would crank but take a few tries until eventually it didn't start at all at a shipper.
There's not a lot of fuel on the fuel/water separator filter. It is brand new.
Mechanics are saying it could be the fuel return valve? But wouldn't that cause for the fuel tanks to go down unevenly?
My engine is a cummins ISX 15 450,
DEF/EGR system deleted/overpassed.