ABYC Diesel Practice Exam

Finding black stringy material in a primary fuel filter is most likely indicative of what?

1. Pump failure

2. Fuel contamination

3. Water in the fuel

4. Old fuel aging

The correct answer is: Fuel contamination

Finding black stringy material in a primary fuel filter is most likely indicative of fuel contamination. This type of material often results from the breakdown of fuel or the presence of contamination such as algae, fungi, or other organic matter that proliferates in the fuel system. When diesel fuel sits for extended periods, especially in a warm environment, it can become a breeding ground for microbial growth. This results in the formation of stringy, black debris that can clog filters and negatively affect fuel flow to the engine. Regular maintenance and cleaning of storage tanks, along with using biocides where appropriate, can help prevent this form of contamination. The other options suggest different issues. Pump failure might present with various symptoms, such as irregular engine performance or difficulty starting, but would not typically cause stringy material in the filter. Water in the fuel may result in rust and sediment but does not typically present as black stringy material. Old fuel aging can lead to viscosity issues and other operational problems, but it would not commonly result in the specific debris found in the filter that indicates contamination.