by Keith Hamburger

Created on: December 22, 2008   Last Updated: February 03, 2009

Water condensation in your fuel tank can cause a number of problems. Ranging from hard starting to stalling to rust holes in your gas tank. To know how to prevent it we need to understand why it occurs.

All air has some water vapor, moisture in gaseous form. Cool air can hold less water vapor than warm air which is why when you have a glass of iced tea on a gold day the glass will "sweat". What is happening is that the cold glass cools the moisture in the air causing the water vapor to condense on the surface of the glass.

The same thing happens when air in your fuel tank cools. The water vapor in the air inside your fuel tank condenses when the sides of the tank cool. That condensate will settle to the bottom of the tank and be trapped since water is heavier than gasoline.

The only practical way to prevent condensation in your fuel tank is to limit the amount of air in the tank in the first place. If there isn't a significant amount of air in the tank there won't be any moisture carried into the tank to condense. Keeping your fuel tank full, especially when humidity is high or you're expecting wide temperature changes will eliminate condensation.

Today, condensation isn't the problem that it used to be. Today's fuel is often mixed with at least ten percent ethanol. Unlike gasoline and water, ethanol and water mix well, and the ethanol will stay mixed with the gasoline. Small amounts of water in your fuel tank, like you would get from condensation, along with ethanol, will be carried into your engine, vaporized and burned.

However, if you're using pure gasoline, with no ethanol, the solution to problems from water in the tank are quite straightforward. Either fill your tank with some ethanol blend gasoline or purchase a fuel additive from you local auto parts store or other retailer. The fuel additives that say they can help with water in your fuel are almost always ethanol based serving the same purpose in such cases as ethanol blend fuels.
Diesel fuel is not commonly blended with ethanol. If you have condensation in your diesel tank you will need to use additives or take additional steps to get it out. Many diesel engines have a bowl with a drain leading to the fuel filter. This is specifically to prevent water from getting to the engine. You should check this bowl periodically for water and, if there is any present, drain it out to prevent it from overfilling and getting to the fuel injectors. There are also additives made for diesel fuel that can help to remove water.

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