induchman

I've been reading about putting an additional fuel/water sep on diesels. Is it really necessary to run one ahead of the oem filter? If yes, where would you put it?

HammerMTB

If you get your fuel at a station that is in regular use, and it's fuel supply is fresh, and has a filter before the pump, an additional pre-filter water separator is unnecessary. When I did a lot of marine service, I saw some nasty problems with water in the fuel. Some were old fuel, some were due to large tanks condensing water vapor in the fuel tank, and some were from "lightering" at sea. That's when a ship gets fuel from another ship. I know you're not doing that.
Point is, if you get fuel from a clean source, an additional filter isn't needed.
We did a test one time to see how much water a filter/separator could stop. It will filter/stop water that is coalesced in fuel, but if straight water is pumped thru the separator, it plugs and passes water. The point here being that if you get a load of fuel that is BADLY contaminated, likely no filter or filters will stop the damage.
So, long story, short input- I don't think they're needed.

Haulin_in_Dixie

The stock fuel filter is a 10 micron filter and is also a water seperator. You won't do much better.

induchman

Thanks. Just putting the co-drivers mind to rest. I've had several boats, including my present tug, which has multiple fuel/water seps in place. Big Racors and a real pain, because of the diversity of fuels out there.