Diagnostic apparatus and method for fuel vapor purge system

Abstract

In a diagnostic apparatus and method for a fuel vapor purge system in which fuel vapor generated in a fuel tank trapped in a chamber is purged into an intake passage of an internal combustion engine through a purge path, a first change in a pressure of the purge path is measured after creating a pressure difference between the inside and outside of the purge path and sealing the purge path, and a second change in pressure that varies with an amount of fuel vapor generated in the fuel tank is measured while the purge path is sealed for a first period of time after an atmospheric pressure is introduced into the purge path. Then, it is determined whether leakage is present in the purge path, based on the first change and the second change in the pressure of the purge path. Before the measurement of the first and second pressure changes, a third change in the pressure that varies with an amount of fuel vapor generated in the fuel tank is measured while the purge path is sealed for a second period of time after the atmospheric pressure is introduced into the purge path before the pressure difference is created. The leakage diagnosis is inhibited when the third change in the internal pressure is greater than a predetermined value, and the leakage diagnosis is permitted when the third change is equal to or less than the predetermined value.