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VOL. 10 NO. 4
Take a class

Fill up your fuel tank

Less space means less moisture

Fill up your fuel tank

We’ve all heard it’s a good idea to keep our fuel tanks full, whether it’s the gas tank in our car or the diesel tank in our boat. Few really know why this is important.

A large quantity of fuel acts as a heat sink, which means the fuel takes a long time to heat or cool enough to match the ambient temperature. While the outside temperature might vary 20 or more degrees daily, the fuel temperature might only vary three to five degrees.

Air taking up unused space in your tank expands and contracts with the temperature change, pushing out through the vent as the day warms and pulling outside air back in overnight as the temperature cools. When this happens, the ambient air entering the tank touches the walls; any moisture in this air condenses onto tank walls and drips into the fuel.

Warm air holds more moisture than cold air. Your fuel is almost always colder than the outside temperature, so any air drawn into the tank will contain more moisture than it can hold inside the tank. That extra moisture ends up in your fuel. Boat engines don’t run well on water, and removing it can be a problem.

The easy solutions is to keep the fuel tank full, leaving little air coming in and out and little tank surface for moisture to condense on. If you have ethanol-blended fuel in your boat, moisture combines with the ethanol and becomes a mess in the bottom of your tank. The current wisdom is to drain the tank for the winter if you have been using ethanol-based fuel in your boat.
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