The factory fuel system on your 2003-04 5.9L Dodge Cummins consists of your underbed fuel tank, fuel/lift pump, high pressure CP3 injection pump, and electronically controlled injectors. For years, Dodge has had a notorious issue with the lift pumps on our Cummins powered trucks. One of the main issues, is its location.
Prior to 2005 the pumps were mounted on the drivers side of the block behind the fuel filter. The two main issues with this setup were that it absorbed a lot of heat from the block, and that it required the pump to "pull" fuel, as opposed to "push" fuel as it was originally designed to do. In model year 2005, Dodge began to mount the fuel pump inside of the fuel tank on vehicles leaving the factory. For owners of 2003-2004 models, if you ever went into the dealership after 2005 with a failed lift pump, they would install a retrofit kit that placed the lift pump in the tank just like the 2005 and later trucks come from the factory. Doing so greatly reduced the lift pump failure rate but unfortunately they are still a low pressure design. This is why we recommend our Fuel Boss mechanical belt driven lift pump or a high pressure fuel pumps from FASS or Pureflow/Airdog.