I have a 502 mag MPI with the Mercruiser cool fuel system and it has always had random vapor lock. It has a standard mechanical fuel pump with a high-pressure pump in the fuel module.
I am rebuilding my engine and was wondering what the best setup is for a fuel cooler and reservoir/high pressure pump?

thanks!

Just plumb the return line back to you fuel tank. End of problem!

Thanks…in addition to the return line, the engine mechanical pump will fill a reservoir, but I need a high-pressure electric pump to the injectors. Its the pump and reservoir I am looking for

You run a mech pump to a res...with an electric pump to feed the engine injectors?

Sounds like an overly complex system for a marine setup. I would be looking to plumb the electric pump from the tank. Then you'll always have pressure to the injectors without having to crank the engine to get fuel with the mech pump.

I believe he’s describing Merc’s VST fuel system . The mech pump was on the sea water pump. It fed the ‘vapor separating tank’ that held the high pressure electric pump. The VST system was used by Mercruiser before (use ended about 1997ish ) the Cool Fuel system came out.

VST

If you have a VST then plumbing a return to tank will be very difficult

"All you have to do is disconnect the fuel return line that goes from the bottom rear of the fuel rail to the VST right where it goes into the VST. You can get a brass fitting from ACE Hardware that will attach to the fuel line. It's inverted flare. The adapter fitting will go to NPT. You can then either go to a hose barb fitting or AN fitting and run back to the tank." -per Eddie Young

I would just get rid of the entire VST system. Use one electric pump with a pressure regulator and then run a return line back to the tank. Use a relay to run the new electric pump.

Thank you all for the input. I do have the Merc system (1997 and before cool fuel).
I am open to doing anything that works. I have a 1996 Donzi 22 BH and I can get to the tank, but don’t fully understand how I would plumb fuel from the tank and back to it? I have the standard fuel out/gauge arm and a vent, but wasn’t planning to pull the tank.
Any suggestions with recommended parts? Thanks again!

Run the return to tank side back to fuel fill hose. They make Tee fittings which will work