I'm working on a boat (malibu responise LX) with a small block chevy engine, EFI, and an inline fuel pump.
After much diagnosis, I replaced the (inline) fuel pump on the premise that a seal had failed and air was getting in. It was losing prime after running for a few minutes.
A new pump, new fittings, reprimed, etc. The new pump pushes gas out into a jar all day long, and also circulates it through the over-pressure bypass all day long. Great, launch the boat, start it, and the pump loses prime.
Prime it again on the spot, it pushes gas again. Start the engine, loses prime.
Is there any difference in fuel supply by powering it blindly with 12v vs while the engine is running? I can't hear the pump running while the engine is running, but that's just because of noise.