The impact of fatty acid methyl ester (FAME) blended diesel fuel on engine/vehicle systems was comprehensively investigated by vehicle, laboratory and engine tests. In this study, 20% FAME blended fuel (B20) was mainly used and soy bean oil methyl ester (SME) was primarily selected as the FAME. Vehicle testing with long-term fuel storage in vehicle fuel tanks was conducted, considering the most severe conditions in market use. Laboratory and engine tests were also conducted to better understand the vehicle test results.In the vehicle test, engine startability, idle roughness and fuel injection control were evaluated using nine vehicles with plastic or metal fuel tanks. All vehicles showed no problems up to 7 months. While five vehicles with plastic fuel tank did not show any problems throughout the test period up to 18 months, four vehicles with metal fuel tanks experienced malfunctions in engine start or fuel injection control following 8, 13, 13 and 18 months respectively. All problems experienced in the vehicles with metal tanks resulted from insufficient and unstable fuel pressure at common-rail of fuel system. The insufficient and unstable fuel pressure was caused by sticking of the suction control valve (SCV) in the fuel pump unit due to compounds accumulated on the SCV needle surface. Ester compound which is considered to be polymerized FAME was commonly observed in the accumulated compounds. Carboxylic acid salt was also detected as a component of the accumulated compounds. In one vehicle, rust was also detected in the accumulated compounds on SCV as well as inside of fuel injector. The five vehicles with plastic fuel tanks demonstrated no problems up to 18 months, though small amount of ester compounds, carboxylic acid salts and rust were detected on the inner surface of the SCV body of one of the plastic fuel tank vehicles.From the laboratory tests, it was found that carboxylic acids, carboxylic acid salts and polymerized SME can be formed when SME-blended diesel fuel was oxidized. Regarding the potential metal sources of the carboxylic acid salts, it was found that not only metal components but also some elastomers used in the fuel system release metals to the fuel.From the engine testing based on an industrial standard test protocol to assess engine torque loss owing to deposit formation, no significant engine torque loss was observed among fuels with various oxidation conditions regarding acid concentrations and Rancimat Induction Period values.