Old fuel
Yours is a FAQ, and justifiably so. Your description of using part of a tank of fuel each year is repeated often. I am right now trying to compose a comprehensive response that is forthcoming. The discoloration in your fuel could come from many things, probably additives. Water is the biggest problem in fuel systems. The design of fuel systems is such that you can have a considerable accumulation of water before any detectable problems. The water usually contains algae. If you add a biocide, you may kill the algae but you dont remove it from the tank. The algae live in the water and eat the fuel. This not only changes the chemistry of the fuel slightly but it introduces the waste from the digestion by the algae. You may add a surfactant that will let some of the water disperse into the fuel. Algae like water and some go along for the ride. Most of the fuel being pumped just returns to the tank, as you state. It goes back to the tank a little warmer than when it left. The algae smile. The magnetic fuel "polishers" do work. These devices have only been around for about 15 years and what they do is hard to believe. When I first got the literature on them it was mainly testimonials that were beyond belief. I was then confronted by a commercial fisherman who had severe contamination in tanks that were built into a steel hull. With a "why not?" attitude, I obtained and installed a unit. The boat went from plugged filters in six to eight hours to not requiring filter changes at all. We now sell the Algae-X units. Some say that the fuel being made today is designed to be consumed within six weeks. Others state that the life of diesel fuel is six months. I personally know of boats that have come out of more than five years of storage that started and ran flawlessly without doing anything to the fuel. There are posts on this forum from sailors who remove all the old fuel and put in new every year. Since your fuel is old, discolored and smells funny, you may want to play with a polisher. Then again, since you are down 30 gallons, this may be the time to remove the old fuel and start afresh. Either way, test for water in the tank and remove any found. Also check for, and repair, the water ingress. It is usually a defective "O" ring on the fill cap. I would like to hear back about what you do and what results you had.
