For the fuel injection system, cracks may develop inside the fuel supply pump, due to an imperfect manufacturing process, and there is a risk that a fuel leak may occur and that the fuel may mix into the engine oil. If this problem is left unattended, the engine oil mixed with the fuel may be sucked into the inlet pipes through the same route as the blow-by gas and be burnt inside the cylinders. If this occurs, the engine revs could rise, or the engine could stop and fail to restart, and in the worst case, the engine oil mixed with the fuel could leak and result in a fire.