maybe its a pickup issue in the tank, collapsing fuel line to the filter head ?? can you switch this engine to the other tank, my dad had a Caddy with a 500 in it and it would do the same thing, turned out to be debris surrounding the sock on the pickup in the tank........I know most boat pickups don't have this sock

Yup, or a sticking anti-siphon valve.

I have removed a few...the damn aluminum housings used by many anti-siphon valve companies + water/ethanol/etc presents some issues...not too mention aluminum tanks that corrode from this and guess what is the first (if no sock) item this passes thru.

Have also dealt with a few cracked pick-up tubes. Fuel level just above crack...all works fine...fuel level get's just below it...it stalls/dies.

This is why BUP always tells people to try running from a separate fuel tank for testing purposes. Also fuel psi just after mechanical pump. If your fuel psi set-up shos vac also, placing this just before mechanical pump can show if there is too much restriction before pump.

so if filled the tank full and it stopped it could very likely be a cracked tube that is something to test for sure. i can also swap hoses coming off the pickups and going to the engines to see if it changes sides if it does then i know its in the tank. if it doesnt then i would try replacing the hoses on that side as it would be cheap thing to do to anyway.

is there anti siphon valve on the pickups?

if none of thst works i wanted to try swapping ecm side to side and map sensors after that if problem doesnt move try a pressure regulator on the fuel rail. beyond that i will need to get a computer on it and th a t might not tell me anything either. they want 120. an hr the min they step on boat and the way i see it bare minimum 3 hrs testing.

Another thing I was thinking about are there any engine sensors that directly make the ecm shut down the fuel pump. Fact is it's a fuel pressure issue without question pressure starts diving after 3500 rpm needle on fuel pressure gauge bounces up and down engine goes to like 1600 rpm fuel pressure will not stabilize and goes to like 20 and lower just thought maybe a sensor is acting up elsewhere and kicking the pump off in vst I am gonna check the other stuff first though from prior post.

Oil psi switch will turn off fuel pump. This is so if oil psi falls under a certain psi (usually low like 10psi) he motor ill stop running.

The antisiphon valve will be the straight barbed fitting screwedd into the 90 deg fitting on your tank. IE: what your rubber fuel line clamps to at the tank.

Some are brass, most I've seen are aluminum. There is a spring and checkball inside of them.

After looking at the check valve ^^^ set up its amazing any fuel at all will flow through there.

well i finally isolated the issue, I swapped fuel lines from side to side off the fuel tank and boom the problem switched sides. so we are going to pull the pickup off and replace what ever needs to be replaced in there along with the check valves. his does not have barbed fittings its all threaded connections with these bigger brass bulb looking things that i assume are the check valves but not sure i have to pull it all apart and see what the deal is.

it was the check valve all gummed up with crap in it cleaned it out checked the other one and boat ran strong

Check vent line before going back to the water , and the fuel line that connects to pump inlet for small cracks .

SB.... nailed that one, well done Sir