Water in diesel fuel is a serious problem that should be addressed immediately. The fuel pump in a diesel engine operates at a very high pressure and with very precise tolerances. Any contaminant, including water, can cause costly damage. Water in the fuel can also promote the growth of bacteria and fungi. These micro-organisms produce a slime that moves through the fuel system, clogging filters and fouling fuel injectors. They also produce acids that can corrode metal surfaces.
Determine whether there is water in the diesel fuel. Extract a small amount of the suspect fuel using a hand-operated bilge pump. Place the fuel in a clear, clean glass container and allow it to sit in a dark place for 24 hours. Diesel is lighter than water so if there is water in the fuel, it will settle to the bottom of the jar.