Mercury Marine’s position is to follow the instructions for normal boat storage found in the operation, maintenance and warranty manual. They believe that if you are planning to store your boat for more than two months, it is best to completely remove all fuel from the tank. But, if it is difficult or completely impossible to remove the fuel, then maintaining a full tank of fuel with fuel stabilizer added to provide fuel stability and corrosion protection is recommended. In this case, Mercury Marine’s website suggests adding the stabilizer and fuel treatment to the tank at the recommended dosage, running the engine for 10 minutes to allow the system to be cleaned, and then shutting off the fuel valve to interrupt the amount of exchange with the air that might bring in condensation. They advise not to cap the tank vent and not fill the tank with fuel to the point of overflowing, as some extra space should be maintained to allow for expansion and contraction of the fuel as temperatures change. And finally, a partially full tank is not recommended because the void above the fuel allows for air movement that can bring in water condensation as the air temperature goes up and down.
As for our 23’ Chaparral with a MerCruiser engine, we have elected to follow Mercury’s recommendation to winterize with an empty tank. It just makes more sense, especially since our winter storage facility is located in southeastern Wisconsin, where the term “polar vortex” may have originated. But no matter what you decide, please check your engine’s owner’s manual for all engine and fuel information. If you do not have access to the owner’s manual for your equipment, contact the engine manufacturer or an authorized dealer. And remember, the EPA’s E15 approval is for vehicles 2001 and newer. If this fuel is used in boats, potential problems can include engine overheating, fuel line ruptures, a breakdown of rubber pieces in engines, expensive repairs and engine failure not covered by warranty!