All fuel injected Vanagons,ie all of them gas and diesel, have two fuel
lines.  One is the feed line that goes to the fuel pump and on to the
engine.  The other runs to the other side of the fuel tank and is the
return line for fuel that wasn't used by the injectors.  The system forms a
big loop and the purpose is to a) simplify controling fuel pressure and b)
to help prevent vapor lock by circulating cool fuel past the injectors at
all times.

Dave K.