It turns over but doesn't start. Will run briefly with a little starter fluid. Then die. Unplug the fuel filter and tried starting it and it has pressure sprayed gas. Fuek pump works when it runs but not when turning it over. Doesn't seem to me that the fuel pump is bad. Maybe the relay? Or a vacuum line? Any advise would be appreciated!

 

If the rubber air tube is disconnected, or broken to where the engine pulls in air that DOESN'T pass through the Mass Air Flow Sensor, then the engine will not stay running. How are you spraying the starting fluid into the intake?
I bought a B2600i from a guy that said the engine would not stay running. Popped the hood, saw the rubber air tube disconnected.......paid him for the truck, got the title, put the air hose back on, cranked the truck up and drove it onto the trailer!

 

Could this be that the electrical part of the ignition switch is not completing the circuit to the fuel pump in the "Start" position, just in the "Run" position? Because they have separate contacts, even though engine still cranks fine. The electrical part of the ignition switch is a common failure area, so I'd test that in the "Start" position.