One of the main contributors to reliability of an engine is the quality of the fuel. You must start off with a good fuel and then not let it get contaminated. Sometimes, but very seldom, the contamination comes with the fuel. Most all fuel dispensers have final filters on their pumps that will shut down the flow if dirty, or if water is trying to get by. The most common contamination is water.
It is virtually impossible to keep water out of the fuel tank. There is always the threat of condensation inside the tank where the moisture in air within the tank is precipitated on the sides of the tank with temperature variations. This can be overcome by not exposing the sides of the tank to the air by keeping them covered with diesel fuel. The usual way for water to get into the fuel tank is at the fill. All fill caps have an O ring to make a seal. If the O ring is defective, the fill becomes a small funnel. Each time it rains or each time the deck is washed, a drop or two of water enters the tank. Over time the accumulation is enough that the fuel pickup to the engine starts to get the water. This usually happens when motoring in a slop where the water forms waves under the fuel and the crests of these waves impinge upon the end of the pickup tube. You don’t know it’s happening until a filter shuts down or, with some filters, the water actually gets to the injectors.
Water in the fuel tank is often discovered when changing filters. If any water is present in any fuel filter, it indicates that there is water in the tank. Now is the time to take care of it. The water can be removed only by picking it off the bottom of the tank. Pumping out all the fuel through the normal pickup tube will not get the water. Many tanks have a hand hole in their tops that can be removed for access. Others have a fuel gauge sender that can be removed. A definite low spot in the tank must be located. If there is no true low spot, one can usually be made by heeling the boat. The water will have run to the low spot quickly. Now you must go in through the access with a pump tube that is stiff enough so that you can feel when it is in that lowest spot and pump out the water. If algae are present, it will take longer to move and all of it may not be pumped out. The filters will find it later if it exists.
There are also wick type devices that can be lowered into the tank that will absorb only water. These can be pulled out from time to time and checked. They get replaced when they are full of water. There are chemicals that can be smeared on the end of a stick and the stick can then be inserted to the bottom of the tank. The chemical changes color if there is water present.
Additives should not be required and, in my opinion, should not be used. If there is no water, no water dispersion medium is needed. If there is no water, there will not be any algae ergo algaecides are not required. Some additives react with others forming crystals. Not good. Some additives alter the combustibility of the fuel and can affect the running of the engine, especially when warm. Some additives kill algae but leave their poor dead bodies in the fuel. Other additives claim to disperse the water into the fuel, when this happens some algae go along for the ride with that water. I have not seen an additive that claims to remove the algae’s fecal matter. The refineries make good fuel that contains the proper lubricating qualities and life enhancing additives. That fuel is distributed to the service stations, truck stops and marinas in good condition. Most dispensers deliver it in good condition. Most contamination occurs with the final user. The final user must keep that fuel in good condition or take the proper steps to remove the contamination.
How does one define “old” fuel? Unless you have completely drained and cleaned your fuel tank, it is probably accurate to state that some of the original fuel is still there. How much it has been diluted (rejuvenated) by new fuel would depend on many variables. If you have a 30-gallon capacity and only use 15 gallons a year, you would cut the old fuel by 50% each year. Considering that many of the larger cruising sailboats have much larger tanks and some don’t burn much more fuel, there is a lot of old fuel that goes sailing. We have seen boats that were in storage for more than five years get launched, new batteries installed, and the engines started as if they had never been idle.
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