I would think that this return line would work I just checked and both vents are 5/8.. Never thought about the fill line,, Will check on one of my 3 am trips the the barn w/ some Joe. Just going to suck cutting a hole in the bulkhead to run the line up through and make everything look neat..

Honestly,, the right way is to just sh_T can the VST and mech. pump and add the Areomotive pump, filters and new regulator so I have enough flow for when I get Eddie Young to build my motors....
I know its about a grand in material per motor just wonder what the labor would be if I let someone else do..?????

most fuel tanks have 2, 3/8inch suction locations. the spare could be used as return. the VST normally returns to fuel filterhead, recycling vapor and heated fuel. mercury doesn't do it so that boat builders can't screw up the 2 lines. you could use dual port valve, similar to diesel or efi trucks. electirc may not be spark suppresed. multi tank could run day(service tank) and use pumps to transfer to that tank. if your having vapor trouble i would do it, cycle key several times prior to startup to flush fuel thru fuel rails. don't use tube to send fuel to bottom of tank, any vapor will linger in fuel and aid vapor entrainment on suction side. anothe benefit is polishing of fuel in tank, continued filtering.

As long as the remaining vent line is 5/8", then no problem.

If you can access the fuel fill then you can use one of these. I just make my own, but these are real nice.
http://www.cpperformance.com/c-500-f...s-returns.aspx

The other option is to add another fitting into the top of the tank like I normally do. Here is a pic. The rubber gasket on this one sqeezed out a little since it was overtightened. This one doesn't look that great but it works.

Eddie.. I like both those ideas, gonna go check that out now...
I'll call you to see if I can get fitting from you. Concerned with metal shavings a little though. Wonder would that be big enought for twins or would I need (2) of those tank fittings?
For everyone else out there in OSO land..
I have talked with Edddie Young many times in the past and I have yet to find any who knows MPI's or EFI's as much.. Cool dude to talk to as well.

you can also get a universal sending unit for a diesel that already has a return port in it and be done with it...simple

How about returning it to the water separator? It would have an input fitting on it. Keeps things in the engine compartment.

Merc did that with the later engines. When they got away from the VST(which returned to the VST up on the intake) and went to the cool fuel setup, they returned the fuel to the filter head. They still had a pretty big problem with vapor locking. Hell, I think it was worse with the cool fuel than it was with the VST.
Eddie

I have a 1998 Powerquest with the same vst systems in my 502 efi/mpi. Did all of those engines have this vaporlock problem? Also, am under the impression that this is happening due to heat from the engine raising the temp of the gas? Is this correct?

until you put in a true return line to the tank it's nothing but a bandaid !!

I run twins so I needed 2 ports.. I have decided to go to the main fuel fill line since its 2" and as Eddie Young shows CP has a connection for this with a bung already on it.. I'm adding another so shouldn't be to hard to install.