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Water is commonly found in diesel fuel due to many factors including condensation in fuel tanks, improper handling and environmental conditions. To minimize the damaging effects, water should be removed from diesel fuel prior to the final stages of solid particulate filtration.

What Causes Water Condensation In Diesel Fuel Tanks?

Diesel fuel tanks are subject to water condensation because unlike gasoline, diesel fuel has no vapour pressure to displace air. When a fuel tank is warm, the air expands and is forced out. As the tank cools at night, humid air is drawn back into the tank and water condenses out on the cooler tank walls. This is one reason to keep diesel fuel tanks topped off if possible.

Additional water can enter a fuel system through an open fill port, a defective tank, or when rain enters a drum that has been left open. Or it can simply be transferred from another tank that is contaminated with water.
Problems caused by water in diesel fuel systems


The presence of water in diesel fuel systems causes a variety of problems. Water causes steel and iron components to rust, forming loose particles of iron oxide. These rust particles can quickly clog fuel filters. Micron sized and smaller rust particles may pass though fuel filters to reach injectors, scoring surfaces and ruining fuel injection spray patterns.


Standing water at the bottom of a fuel tank provides an excellent environment for a wide variety of soil bacteria to flourish. These bacteria enter through tank openings and during dispensing. Fuel and water form an interface that provides a comfortable home where bacteria can feed on diesel fuel in a moist environment. These bacteria form a slime layer that often breaks free to rapidly clog fuel filters and disperse bacteria throughout the fuel system. A fuel filter that is plugged with bacterial slime will have a dark layer of wet gel on the upstream surface. Living bacteria pump out acids as a waste product, further corroding and damaging fuel system components.


Finally, any water that makes its way through to the fuel injectors reduces the lubrication properties of the diesel fuel along the way. This leads to galling, premature wear, damaged injection tips, and further corrosion of sensitive fuel system mechanical components.

When Is The Best Time To Remove Water?

When diesel fuel contaminated with water is pumped from a fuel tank, the water leaves the tank as visible floating droplets which are relatively easy to remove from the fuel using a quality fuel/water separator. If the water is not removed prior to passing through the pump, the diesel fuel/water mixture forms a stable emulsion, making it very difficult to remove the water. Water dispersed this way may pass through the fuel system to be burned, but more likely some will settle in, and corrode sensitive components.

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