What's the most likely cause?

[Fast Leroy]

So my 3.5 Eco boost fuel rail pressure leaks off after the truck sits for a while. The low pressure fuel pump then sometimes can take 3-4 minutes to pump up and shut off before the truck will start again. If I don't wait until the LPFP shuts off it won't start. I believe the pump is ok because the truck runs fine after it starts... What's the most likely cause?

[mechanic416]

Pressure regulator or fuel injectors are leaking down.

[Fast Leroy]

Pressure regulator or fuel injectors are leaking down.

Thanks, any way to troubleshoot to know which for sure?

[CAVEMANN]

Thanks, any way to troubleshoot to know which for sure?

Install a test gauge to the system while running & then kill the engine & keep an eye on the gauge, not really much else than I can think of that would cause it.

[farmersammm]

There should also be a backflow preventer/check valve in the fuel pump IIRC. If it fails, it will bleed back down into the tank. But it should also exhibit hard starting, and possible low fuel pressure too.