Why does the fuel appear to be dark or black in color?

In some Lambert engines, normal operation can cause the diesel fuel in the engine and fuel tank to appear dark or black in color. Discoloration of the fuel can be caused by the following:

·         Engine lubricating oil mixing with the fuel during operation

·         The formation of asphaltenes

·         Degradation of fuel from storage

Darkening of diesel fuel due to the mixing of lubricating oil or asphaltene formation does not indicate a manufacturing defect or other warrantable malfunction, and is a function of normal operation. Customers should continue using the equipment as is, unless the discoloration has resulted from fuel overheating, fuel contamination, or if fuel filter life is being reduced and causing operational difficulties with the equipment.
 

Mixing of engine lubricating oil: In some fuel systems, engine lubricating oil and diesel fuel are used in close proximity to each other for lubricating and sealing purposes. This interface is a function of fuel pump and/or injector design. Under certain operating conditions, a small amount of lubricating oil can mix with the diesel fuel and be returned to the tank, causing the fuel to appear dark in color. It takes a very small amount of lubricating oil (less than 0.1 percent) to cause the fuel to become visibly darkened. This small amount of oil in the fuel will have no adverse effects on engine performance, durability, or reliability.

Asphaltene formation: This phenomenon is common for fuel systems that operate at very high fuel pressures and temperatures. The high temperature fuel that is not injected into a combustion cylinder is returned to the fuel tank. As the fuel is recirculated and exposed to the same high pressures and temperatures during continuous operation, asphaltenes begin to form larger clusters of insoluble materials that can lead to fuel discoloration. If these formations of asphaltenes grow large enough, they will be captured by the filter element and can lead to high filter restriction and shortened filter life.

Degradation of fuel from storage: Referred to as “Microbial Contamination”. The growth of a large colony of microorganisms in a fuel system can cause several issues. The first and usually most obvious is fuel filter plugging with a greenish-black or brown slime, frequently accompanied by a foul odor. This slimy, string-like colony can also plug sharp bends in fuel lines, fuel meters, and other restrictions. The second issue these microorganisms can cause is corrosion due to the acid by-products some of them produce. It is also possible, if the microorganisms pass through the fuel filter, that they will form deposits and cause damage in the fuel pump and injectors.