Mercruiser Mag 350 Vapor Lock? Something Else?

OK....so the last three times i've been out the boat fires up and runs great. As soon as I cut the motor and five or ten minutes go by it only fires up for about three seconds and cuts off. There is a distinct sound of air being sucked into the motor. When I push the attached stem to test the fuel pressure lots of air comes out! Normally fuel would bleed out instead of air. Its as if the fuel is being displaced by air. Once I let all the air out and turn the motor a few times is tarts and runs great again...any ideas?

You know vapor lock happens on the fuel feed from the tank to the pump. Usually that line is getting restricted somehow and there is too much negative pressure in that line after the restriction. When you shut off the pump, the vacuum holds and causes the fuel to evaporate quickly. You might want to check for kinks, maybe even remove the fuel supply line to the pump and blow some compressed air or replace it. The fuel line could also be getting hot somehow.

Thanks.....so would it be reasonable to say the fuel cooler where the fuel line passes through would need to be checked as well.

Yeah check that fuel cooler and feel the entire line to see if there are any hot spots. You can run your blower to cool the engine compartment too.