WHAT CAUSES WATER CONDENSATION IN DIESEL FUEL TANKS?
Diesel fuel tanks are subject to water condensation because unlike gasoline, diesel fuel has no vapor 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.