1) I got an 04 2500 dodge cummins that has been awesome till now.
It started with the truck dieing as if it was running out of fuel. the fuel filter was black when i had change it and didnt start after.
I then found out that my fuel pump had took a dive on me.
so the dealership toldd me i had a intank lift pump
2) i purchased a new intank fuel put and dropped the tank and installed the new one.
3) after that i discoverd that the stock one wasnt a intank pump it was just a fuel gauge.
4) i left the intank fuel pump in cuz i had a full tank and installed a new FASS 95 series fuel pump.
5) when i first started it it seemed slugish but got normal after driving it through a 1/4 tank of fuel.
6) when i had got to a 1/2 tank it had took a dive on me again
7) i then fund that i had run out of fuel at 1/2 tank.
8) after i filled it up.... it only took 22 gallons and i have a 35 gallon tank it started right up.
9) but now it seems slugish again and still have a full tank.

any ideas could be a huge help!!!! is this any easy fix or a hole in my pocket?

 

Try Taking Off Your Fuel Line From Your Cp3 And Running It Into A 5 Gal Fuel Can To Check Flow.

 

Did you take off the fuel pump on the engine? Are you running two fuel pumps at the same time? One in tank and the Fass. Fass has a straw for the fuel tank, is it all the way to the bottom of the tank? The fuel may be going through too many pumps.