This study proposes a method to alleviate fuel discoloration and diesel bug contamination occurrences in K1-series tanks. A continuous illumination of the fuel water separator filter blockage warning light was detected during the field operation of K1-series tanks. A field test evaluation was conducted to analyze the cause of this issue, and black discoloration of diesel fuel was detected. Investigation was conducted to study similar cases of fuel discoloration, and examples were identified in commercial vehicles in which engine oil leaked into the fuel tank during the operation of the fuel injection pump. Research was conducted to identify the cause of the fuel discoloration phenomenon occurring in K1-series tanks, and to analyze the effect of diesel bug contamination on filter blockage. Improvements in the maintenance procedure and fuel system flush were proposed to alleviate fuel discoloration and diesel bug contamination.