Failure diagnostic system of evaporated fuel processing system

This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2001-156808 filed in Japan on May 25, 2001, which is herein incorporated by reference. A failure diagnostic system reduces the internal pressure of a fuel tank to a predetermined negative pressure, seals off the fuel tank from the air, and then determines whether an evaporated fuel processing system has failed or not by monitoring an increase in pressure in the fuel tank. In determination, the failure diagnostic system compares a detected pressure in the fuel tank with a reference value that is increased by a predetermined rate. The failure diagnostic system stops update of the detected pressure if the detected pressure becomes higher than the reference value, and resumes update of the detected pressure if the detected pressure becomes lower than the reference value. It is therefore possible to accurately determine whether the evaporated fuel processing system has failed or not even if the pressure in the fuel tank is rapidly increased.