500 Efi fuel pressure regulator?
 

I have a 5000 EFI that has a spot on the back of the plenum for a single vacuume hose that goes to the regulator by the fuel pump and the regulator on the rail has a rubber boot on it and on my buddy's engine he has a spot for 2 hoseson the plenum one to the regulator on the fuel rail and the other on the fuel pump should I run a hose to the one on my fuel rail as well or should I leave it blocked off

Any help would be appreciated

hook it up like your buddy's, thats the way it is from the factory

I also have 3 other 500 Efi's sitting on the shelf in my garage and all 3 have the little rubber boot on the regulator on the fuel rail I just don't understand how it could be doing anything

maybe they were running aftermarket fuel pumps on those & not cool fuel in this case not needed, unless you are running a bypass regulator like I am and in this case the vacuum hose is connected to the regulator

#20 is regulator on fuel rail
http://www.mercruiserparts.com/images/COMMON/9917.png

#15 is regulator on fuel cooler

http://www.mercruiserparts.com/images/COMMON/9918.png

So...looking at the diagrams, the regulator on the fuel rail has the return port blocked off, making the regulator ineffective. ie: not doing anything.
#18 and #19 in first diagram are a block off and cap.

THe regulator on the cool fuel, does return, therefore this regulator is what is creating your fuel pressure.

As a note: I am going by the diagram and my knowledge of fuel systems. I have not worked on these 500 EFI systems, therefore take my words with a grain of salt.

I think they were using the one at the end of the fuel rail as a buffer.

The fuel rail one is really a FUEL PRESSURE DAMPER - the one in the cool fuel is the real fuel pressure regulator.

The fuel rail FP damper vacuum line goes to the fitting on the back of the upper intake plenuim as the same area for the vacuum line for the fuel pressure regulator (cool fuel one)

the fuel rail fuel pressure damper should have a neck on one end of it to connect the vacuum line to.

SB can you post the parts list to your first diagram of the fuel rail

Merc always has listed # 20 as a regulator but really is called fuel pressure damper. If I recall the OEM Merc Racing service manual calls it the fuel pressure damper thru out in the book as well. That would be manual # 6. If I pull it out or find it I can double check all of it but I am 99.9 % I am correct of all what I posted in above.