Lately as soon as i get onto a main road and drive around 50 mph the water in fuel light comes on. The first time it happened i kind of panicked and let off the pedal, looking for a place to pull over, but the light went off; so i then drove some more, put my foot on the pedal some more and it did the same thing until i let go somewhat. Usually by this time i then hit a switch on the dash and start drawing fuel from my secondary tank of heated veg oil, then i don't see the light anymore even when i switch back to the main tank before shutting down.
After the first time this happened i drained the bottom of my filter (the box-shaped stock 6.2 filter) but found nothing but fuel - no water or crud.
Although the fuel i run in that tank is homebrewed biodiesel which has been mist washed, it woud not surprise me if i did find some water in the filter but it has not happened.
My van has lots of quirks with the electrical system, there seems to be ground issues and old, corroded wire terminals i am always fixing.
Is it possible that this is just another such quirk? How to figure out if i really am getting water in the fuel when none comes out the drain of the filter? If it is just an electrical issue, any advice on how to solve?
thanks,
Paul

The "Water in Fuel" sensor is only part of the equation. There is a pressure sensor located on the back of the filter housing. (black button w/wires coming off of it) It, too, is wired into the same light. As pressure rises, the filter pushes out and contacts the sensor and turns on the "Water in Fuel" light. This pressure rise occurs as the fuel filter becomes restricted, either by water or foreign material. Change the filter and the problem should go away.

I'm leaning more toward it being an electrical problem.

My truck used to do something similiar.
WIF light would come on and the speedo would cut out just for a few seconds every once in a while. I've fixed more than my fair share or electrical problems with my last few vehicles I've owned. Seems I always buy trucks from guys who will never be electricians.

My truck does the same thing when the fuel filter gets plugged, with water or debris. a new filter from napa is $15 bucks. do this before thinking about electrical. Now if the wif light comes on at random It could be electrical, but at high speeds or under hare accel, it is prolly the filter.