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Abstract

A highly accurate diagnosing apparatus for an evaporation purge system, which is not so easily affected by fluctuation in an atmospheric pressure. An evaporation system including a fuel tank is set in a pressure state different from an atmospheric pressure, and then hermetically sealed during leakage diagnosis. A reference pressure is introduced to the reference pressure introduction side of a relative pressure sensor, and a pressure in the evaporation system is introduced to the detected pressure introduction side thereof. In an atmosphere introduction passage for opening the reference pressure introduction side of the relative pressure sensor to atmosphere, a solenoid valve is provided for the relative pressure sensor. During the leakage diagnosis, a control unit sets the solenoid valve in a closed state, holds the reference pressure at a constant value, and then determines the presence of leakage in the evaporation system based on a relative pressure change detected by the relative pressure sensor.

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First Claim