Got a new to me boat with low hour 2001 350 Mag MPI's with vapor lock issues. At first it was just the port engine giving me problems so I replaced the pump and cooler and all seemed well until today. Now both engines are vapor locked (it is a lot warmer than it has been today). Went to lunch, kept the hatch cracked open, had no problems restarting. go back home to grab a few things, crack open the hatch come back 20 minutes later and both are vapor locked. I have had the hatch open for three hours and it's still not cool enough for the engines to start back up and run. Is running a return line the only fix? I have no provisions for return lines in my fuel tank. What are my other options? Also there is 60 gallons of ethanol fuel in the tank that I put in, I don't know what was in from previous owner. Thanks

There is a way to run a return to the tank, it goes back to the boats gas fill hose using a Tee fitting.

To get you going on the spot, have a rag, safety glasses and small screw driver.
Put on safety glasses and remove cap off Schrader valve.
Put rag over valve and use screw driver to press in on valve.
Turn key ON/OFF a couple times and bleed air out of fuel rail

Once air is out and have only gas, start the motor

So I will have to research finding a tee fitting to tie both engines into that line (single tank dual pickups).
I tried the method above but must have not did the proper procedure, I had a rag and hit the valve until no more fuel or air came out, turned the key to on a few times to get the pump running and pressure but it did not work, engine started but only ran a few seconds,

Fuel psi guage testers have a push release psi valve connected to short length of tubing. You csn release into bottle or something. Cleaner and no fuel spilled.

Yep, I was using my Fuel psi gauge in my initial troubleshooting with the port engine vapor locking. I think I was too mad to grab that today to try and drain the fuel rail LOL.

Something like one of these:
https://www.cpperformance.com/c-500-...s-returns.aspx

Awesome!! Thank you!

Just a heads up but not all the small block applications have the Schrader valve. And to throw another wrench in the problem on those motors with the combo mechanical fuel pump and VST, when the mechanical fuel pump gives up the ghost the motor will have problems very similar to vapor lock. Had this happen last week.

If you connect 2 return lines to 1 to return to the tank, is there a risk of gas being pushed from one engine, against the other? Not sure if it would become a problem, but thinking 1 line per engine return would be better.

just a thought.

No expert but thinking with open fuel lines there should be very little pressure if any in the line