Very low fuel pressure

I have adapted three GM S-10 throttle bodies to a flathead Lincoln V-12. We think it looks cool. We're using a FuelTech ECM this is pretty intuitive. Problem is fuel pressure drops from 10 psi to three psi when started. I may have done this to myself with a surge tank and a carburetor style lift pump slightly under the gas tank. Fuel goes into the surge tank on the top, and out the bottom. Also connected to the surge tank bottom is the return line. A vent line to the gas fill pipe is on the top as well. A stock style Carter TBI fuel pump and filter are mounted in the frame rail. We took the springs and the gasket center out of the built in regulators and are using an Aeromotive bypass regulator in return from the TBI bodies. The regulator also has a pressure gauge.

Had to prime the TBI pump to get fuel flow, and then had to use three washers in the regulator spring chamber with the stiffest spring to get 10 psi on the gauge with the engine not running but both pumps on. All lines are 3/8. Wondering is the overflow line to the gas inlet at the gas tank is too low resistance to flow relative to the line through the pump and fuel injector bodies? Engine starts fine, but then the ECM gives a low pressure alarm and the engine dies.

All advise is welcome
AbeAttachment 16859

line size probably isn't the issue if it's dropping like that even at idle. a 3/8 line should flow enough fuel to idle a dozen engines. if your pressure is dropping on a non vacuum referenced regulator system at idle it really can only be at or before the pump (failed pump, supply voltage, cavitation, something like that)

i'd like to see a drawing of your plumbing layout

Mine is similar to this, only return pipe goes to bottom of surge tank.
Thanks
Attachment 16865

it's weird to me to have a surge tank without putting the pressure pump inside of it (quieter and cooler, that's why all car manufacturers put them in there)

to be clear which return is at the bottom? for the high pressure loop, or the fuel tank return? your fuel tank return should definitely be at the top to ensure maximum filling of that surge tank.

is your regulator vacuum referenced?

Return is to bottom of surge tank. I could put a T in the top bung to share bung with overflow tube. Regulator is open to air per Aeromotive recommendation. Thanks for the advice.
Abe

i would prefer to see it like the picture with everything at the top. the way you have designed it sounds like the kitchen sink with the drain open which is the worst possible design. it'll never fill. you really want that thing to be completely full or the returning fuel from the high pressure pump will aerate and eventually you'll just be pumping froth. put your feeds at the bottom and returns at the top.

this might not be your problem but it's worth making sure it's ideal

it's way better to just chuck the pump in the bottom of the tank

Have you checked the voltage at the EFI pump and compared readings with engine running and key on not running? Maybe its dropping when the engine is turned on. Whatever it is as KOEO (key on engine off), it should be higher with engine running since the alternator / generator comes into play