I have changed the fuel filter twice, cleaned the prefilter twice, and replaced the overflow valve. I have installed a fuel pressure gauge and I have 10 psi at idle and by the time I get up to 40mph its at 0psi. I partially plugged the return line and the fuel pressure would occasionally surge from 30psi to 50psi at idle but stayed on 30psi most of the time. When I took it for a test drive by the time I got up to 50mph the fuel pressure would be back down to 0psi. After stopping it would take about 30 seconds for the fuel pressure to come back up and when I drive it would go back down to 0psi. Is this a confirmation of a bad lift pump? If so where is the best place to buy one?

John

 

Sounds like a bad LP alright.
I have a good used one for cheap if you want to go that route PM me.

 

If your gauge is electric it could be the sender, otherwise lift pump.
$79 from Cummins.

 

Lift pump part number 4944710 from cummins.