The Ethanol Debate

Leave your gas tank full or empty over the winter?

By Charles Fort
Published: October 2013

Photo of a gas pump
If you have ethanol in your gas, you need to take steps to protect your boat over the winter.

Ever since E10 (a gasoline blend with 10 percent ethanol) became widespread, there have been two schools of thought when it comes to winterizing your boat: Keep the tank full, or empty it completely.

Both sides agree on the basic facts. Ethanol is hydrophilic — it attracts and holds water. Small amounts of water will be burned in the combustion cycle, but ethanol can only absorb so much water. When it becomes completely saturated, phase separation occurs, and the corrosive ethanol-water mixture sinks to the bottom of the tank. If the engine is run, this mixture can damage seals, O-rings, injectors, and other delicate engine parts. The upper "gasoline" layer will be depleted of ethanol and have a reduced octane level, which can also cause engine problems. If a tank is left partially full, there is more room in the tank, which means more surface area for condensation formation and less ethanol to absorb the resulting water. Cycling temperatures from warm to cold also increases the amount of condensation. Finally, ethanol cannot absorb as much water at low temperatures as it can when it's warmer, so the gasoline will phase separate more quickly in colder temperatures.

The obvious answer is to completely empty the tank — and even the fuel lines and filters if possible. But any boat owner with a gas tank capacity of more than a few gallons will tell you how problematic it is to completely drain it, and some indoor storage facilities don't allow empty tanks because they may contain volatile fumes (usually there is an exception for portable tanks that are empty and uncapped). USCG regulations prohibit drain valves on the bottom of a gas tank (imagine what would happen if one corroded through or broke off, leaving a bilge full of gasoline) so the only way to remove gas from a large tank is by pumping it out. Too many of our claim files prove that using something like a shop vac to do the job will land you in the hospital. Finding ignition-protected equipment and transporting and disposing of the gas afterward is simply not practical for the average boat owner.

That's why Seaworthy has always been in the keep-it-full camp. We are, after all, boat owners, so we're forced to take our own advice. If it's impractical or impossible to empty the tank completely, then keeping it full should make it more difficult for enough water to get into the fuel to cause problems. But just in case we missed something, we asked some industry experts for their thoughts on the subject.

Bob Popiel, head of marine service at Yamaha, said his preference is complete draining of the fuel system including tank, fuel lines, carburetor bowls, and the vapor separator tank (VST). VSTs, he says, can hold a cup of fuel and they're vented to the atmosphere, as are carburetor bowls. The VST has a drain, but it is hard to access for a typical owner, while on older engines, carburetor bowls have drains that most people can get to, he says. Draining the system is even more important on older engines (10-15 years old) because they don't have the same ethanol-resistant parts that new engines do. That said, Popiel knows that draining the whole fuel system is often difficult, so another option is to simply disconnect the fuel supply and let the engine run out of fuel, which will at least drain most of the gas from the VST or float bowls. If you choose not to drain the system, he recommends adding a good quality stabilizer to fuel, topping up the tank, and running the engine enough to get fuel all the way through the system. Ten minutes ought to do it, he says.

John McKnight, director of environmental and safety compliance with the National Marine Manufacturers Association, says, if you're storing gasoline for two months or more, try to drain the tank if possible. If it's not practical, add stabilizer, fill the tank, and run the engine for 10 minutes to allow the treated gasoline to get into the fuel system.

Mercury Marine says that if it is difficult or not possible to remove the fuel, maintain a full tank of fuel with a fuel stabilizer added to provide fuel stability and corrosion protection. Top off the tank until it's full to reduce the amount of exchange with the air that might bring in condensation, don't cap the tank vent, and don't fill with fuel to the point of overflowing so there is some extra space in the tank to allow for fuel expansion and contraction with temperature changes.

So there you have it. If you can empty your tank — and entire fuel system — for the winter and your storage facility allows it, then do it. But where that's impractical, keep the tank full, add a stabilizer, run the engine for 10 minutes, and sleep well.End of story marker


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