Ask SAIL: Using Fuel After it's Been Stored

I have an 8hp two-stroke outboard engine on my sailboat. I use a 5-gallon portable gas tank and always mix water-absorbing additives and the required lube oil into my fuel. The problem is at the end of last sailing season, I had about four gallons of fuel left in the tank. Is it okay to use this fuel after it has been stored on the boat over the winter, or should I dispose of it and get fresh gas instead?
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Gil Dillon of Whitestone, New York asks:

I have an 8hp two-stroke outboard engine on my sailboat. I use a 5-gallon portable gas tank and always mix water-absorbing additives and the required lube oil into my fuel. The problem is at the end of last sailing season, I had about four gallons of fuel left in the tank. Is it okay to use this fuel after it has been stored on the boat over the winter, or should I dispose of it and get fresh gas instead?


Nigel Calder replies:

The underlying problem here is that ethanol, which is now in all gasoline, absorbs any water that gathers in a tank, primarily from condensation on tank walls. If enough water is absorbed, something called phase separation occurs and a corrosive ethanol/water mix settles at the bottom of the tank and remains there even after fresh fuel is added. If this is drawn into the engine, it can cause damage.

As far as I know, there are no additives on the market that will prevent this. To avoid these problems, fuel tanks should either be completely emptied at the end of a season or topped off to minimize condensation on tank walls. Your tank has been kept pretty full, so you are unlikely to have a problem. You might try to draw a sample from the very bottom of the tank to confirm this. If you do have phase separation, you should get rid of all the fuel, not just the stuff at the bottom of the tank. The phase separation of the ethanol will have lowered the octane rating of the fuel, which may also damage the engine.

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