Agree with Dave.
Look for a low point in the vent line that traps oil - especially when
the boat is over on her ear.
Other possibility is thay the 'fill' nozzle mounted ON the tank in the
direct center of the top and the inlet flow of fuel promotes too much
air to be entrained into the fuel causing an over abundance of foam
being produced. (A good tank will have the fill nozzle discharging onto
the side of a tank and well below the top to prevent such foaming).

If your problem is 'foam' coming out the vent simply consider to fill
the tanks more slowly, or put a 'bubble trap' inline with the vent ---
think of a 'pitot tube' mounted inside the vent line.






In article , Michael Porter
wrote:

Are you absolutely sure there is no low point in the vent line? Sounds
as though one possibility is diesel collecting somewhere and being
blown out by the filling pressure.



"Dave Isherwood" wrote:

Boat is a Carver 530 Voyager and with two vents on either side. When I fill
(even using a normal slow pump) I get diesel fuel venting. Yesterday I had
to add 200 gal. to each side and venting started after I'd put in 10 gal.
I'm at a lose to understand why this is happening. Foaming seems unlikely as
it would need to have filled 50% of the tank!

Any ideas of what is going on? Or even better a simple remedy that works!

Thanks,

Dave

Michael Porter Marine Design
mporter at mp-marine dot com
www.mp-marine.com