I am going to be converting my engines over to EFI this spring. My dilemma is how to plumb it. My boat has 3 fuel tanks, a belly mounted 125 gal and two 70 gal saddle tanks. The fuel is pulled thru fuel water separators then thru a low pressure fuel tank selector valve ( a two port 12 V low pressure solenoid valve that looks similar to a nitrous fuel solenoid) buy the engine mounted fuel pump. How to other boats that have multiple tanks do it?

I have three tanks and fuel flow meters, so there is no practical way I can use a return line to the tank during my conversion from carbed to EFI. On the first try I installed a stock Merc MPI system with the VST tank. Then I experianced the infamous vapor lock that that system is known for. Next I eliminated the VST and ran the return line to the suction side of the low pressure pump. This almost cured it, but I was still having issues if I idled for a long period. Sometimes it would be hard to start, but at least it started. The next step I took was installing a fuel cooler. Which was a power steering cooler that I simply ran fuel through instead of PS fluid. After the fuel cooler was installed all my problems seemed to disappear until I went to lake Powell. The combination of 3,500' altitude, E10 gas, and 95 degrees temps allowed vapor lock to raise its ugly head again. We added a couple of gallons of Marvel Mystery oil and it helped alot. But we still had to be careful where we turned the boat off. Because we were'nt sure it would start in an hour or so. I have recently removed all the Merc stuff and installed the Holley HP EFI. I'm still using the fuel system that I assembled for the Merc MPI. So far so good, but I have only had it out a few times.

That's only part of my EFI conversion story, but it should give you an idea of what you are in for.

Is the issue return lines?

Wild Thunder? You wanna take this one? You LOOOOOOVE EFI I hear

Who's efi?

FiXX- I am going to be using a DFI system.
Motor- yes the concern is the return lines.

Here is a idea of how to do it. Using the existing carb fuel pump, dump that into a swirl tank from which will feed the electric pump. Have the return from the fuel rails feed back into the swirl tank. And have the overflow from the swirl tank going to a 3 port selector valve which then dumps back into what ever tank is being drawn from.

I wouldn't dump the return fuel into the swirl tank. You're better off dumping it right back to the tank it came from. Otherwise you are heating and aerating the fuel.

X2

If your system has the capability to control the fuel pump voltage and pressure with a pressure sender you may not need the return.
This is how the MSD systems work.

Jeff