Recently I emptied a heating oil tank into a half full barrel of old bio-diesel that I had. The heating oil was only about 8 months old though. The reason the bio was old is because my pump broke and I couldn't pump it into my truck anymore. That is until I got the heating oil and could spare a few buck to get a new pump. I put one tank of the bio/heating oil through the truck with no problems. now on my second fill up the little fuel with rain drops light is on. I've haven't driven it for the past few days because I want to figure this out before I ruin an injector or something. I do have a water seporator/filter on my pump set up so i thought I would be ok. I guess not. What are my options now, I can drain the tank but is that necessary or can I add an additive to bond to the moisture?

 

Was the biodiesel tightly sealed and properly made? Biodiesel is hygroscopic and readily absorbs moisture. Those water separators do nothing for suspended moisture. If it were me, I would drain your tank and fill with fresh pump diesel. Not enough information on your old biodiesel to give advice on that. I would suggest you take a sample of your heating oil/biodiesel blend and test for water content. It should be below 500ppm. WHW Test for Water | Quality Testing

 

If that dash light is on there should be water settled in the bottom of the fuel bowl, completing the circuit that energizes the light. The fuel bowl and yellow drain on the back is there for this purpose. Drain some out into a container and you'll probably see droplets of water rolling around the bottom. Lather rinse and repeat until no more water, keying the pump on as necessary. Fill the tank with diesel to dilute, and keep an eye on the WIF light until that tank is gone. Drain more along the way if it makes ya feel good.

I'd guess the truck will be fine. But I would not use more of that fuel without seriously diluting it with diesel and draining it often.

There are fuel additives that break up the water into small bits and theoretically makes it safe to use. I have no science to back it up but I just don't buy that idea. I'd rather the water stay chunky and drain it from the bowl as intended.