For the mechanical type fuel injection pump for the diesel engine, the spring for the fuel supply valve may not be of the right type because the assembly instruction for the parts supplier was not proper, and this may cause the fuel supply timing to be different. As a result, black smoke discharge may increase at certain engine revolution rates. In addition, the exhaust gas recirculation system may not be in the right condition, because the vehicle dealer, who was given an improper maintenance instruction, carried out inappropriate maintenance.