Although emission regulations are becoming
increasingly stricter, there are still contaminants
in fuel that are within the American Society for
Testing and Materials (ASTM) and the American
Petroleum Institute (API) specifications.

We must all realize that when filters plug up, slowing
fuel flow or stopping it altogether, they have done their job. When they seem to last a short time, it is simply that there is too much particulate, wax, gelled fuel, water, etc.,
in the fuel and the filters are removing it. When this
happens prematurely, we must take steps to recognize and
correct the problem.

Fungus and Bacteria are the most widespread
contaminants in today’s diesel fuels. These fungi and
bacteria live in the water and feed on the hydrocarbons
found in the fuel. These contaminants are called
Hydrocarbon Utilizing Micro-organisms, or Humbugs, for
short.

A microbial problem is evident when the spores
become active and multiply, forming colonies and mats of
growth. Colonies will spread through a fuel system
wherever moisture or trace amounts of water are present.

Bacterial problems are characterized by short filter life. The fuel filter will have a slime over the entire surface of the media. Draining the fuel system will reduce microbial activity, but it will not eliminate it. The only way
to eliminate microbial growth, once it has started in a diesel fuel system, is to treat the system with a biocide.

Precipitants and Particulates are non-combustible materials formed when two incompatible fuels are mixed, or when fuel oxidizes. Tests suggest that fuel can oxidize when an unstable fuel circulates through an engine. Heating and cooling occur when the fuel used for cooling the injectors is returned in large quantities to the tank,
causing the oxidation.

Asphaltines sometimes can be
formed as part of the oxidation process. Asphaltines are uncracked or damaged molecules left over from the refining process and are found as an oily, black substance on the surface of your fuel filter media. Most of the time, these contaminants will settle out in low or slow flow areas of the fuel system or become part of
the tank bottom sludge. Both the precipitants and the
particulates do get stirred into the fuel system during fuel deliveries and can plug filtering systems.

Water in fuel probably causes people the greatest
concern, because it is the most common form of diesel fuel contaminant. Water is found in your fuel system in two forms, free and emulsified.

Most diesel fuels have some dissolved moisture. Diesel
fuel has a saturation level of water at any given
temperature. As the temperature goes down, the fuel will hold less dissolved moisture. Any moisture above this saturation level will not dissolve, but will fall out.