Fungi and other microorganisms can survive and multiply in diesel fuel if water is present. The fungi can be present in any part of the fuel handling system. These fungi grow into long strings and will form into large globules. The growths appear slimy and are usually black, green, or brown. The fungi may grow anywhere in the fuel but are most plentiful where diesel fuel and water meet. Service station tanks may contain fungi that could be pumped into a vehicle during refueling.

Fungi use the fuel as their main energy supply and need only trace amounts of water and minerals. As they grow and multiply, they change fuel into water, sludge, acids, and products of metabolism. The most common symptom is fuel filter plugging, however various metal fuel system components including fuel sender assembly, pipes, fuel injectors, and fuel injection pump can corrode.

Warning: Avoid physical contact with the biocides in order to avoid personal injury.

If fungi have caused fuel system contamination, use a diesel fuel biocide to sterilize the fuel system. Do not exceed the dosage recommended on the label. Discontinue the use of a biocide when towing a trailer due to possible engine power loss. It is permissible to have biocide in the fuel when starting to tow, but do not add any biocide while towing.

Water in fuel will also create drivability concerns and loss of engine power. If water is present in the fuel system , the fuel cannot cool and lubricate the components, causing overheating , rust and corrosion. This can result in component failure. Water can enter the fuel system in several ways, either through a contaminated refueling source, or through long term condensation the vehicle fuel tanks.

Like fungi and water, gasoline contamination will cause drivability concerns and a possible hard starting or a no start condition if the overall content is large enough. Gasoline raises the API rating and reduces cooling and lubrication, resulting in possible component failure.

Water in fuel will also create drivability concerns and loss of engine power. If water is present in the fuel system , the fuel cannot cool and lubricate the components, causing overheating , rust and corrosion. This can result in component failure. Water can enter the fuel system in several ways, either through a contaminated refueling source, or through long term condensation the vehicle fuel tanks.

Like fungi and water, gasoline contamination will cause drivability concerns and a possible hard starting or a no start condition if the overall content is large enough. Gasoline raises the API rating and reduces cooling and lubrication, resulting in possible component failure.

It is acceptable to use diesel fuel containing up to 5 percent bio-diesel. Bio-diesel is produced from vegetable oils or animal fat that have been chemically modified to reduce the possibility of damage to the fuel system and engine components. Concentrations greater than B5 fuels or the use of unmodified bio-oils blended into diesel fuel at any concentration is not recommended and may cause drivability concerns or component failure.

Inspecting for Fuel Contaminants

1. Test the fuel specific gravity and record the results. Refer to Fuel System Specifications.
2. Remove the fuel filter and inspect the inside area of the fuel filter for dirt, water deposits, or other debris.
3. Drain the fuel filter into an approved clear container and allow the contents to settle. Observe the container for contaminant separation.
4. Remove the water-in-fuel sensor from the bottom of the fuel filter and inspect inside the filter for rust deposits, dirt, or other debris.

Note: The presence of water or gasoline in diesel fuel may also cause damage to other fuel system components.

5. If contaminants are present in the fuel or in any of the inspected areas, remove the fuel lines to the injectors and inspect the fuel injector return outlets and feed inlets for rust or corrosion.