To diagnose the leakage of an evaporation system with high accuracy by detecting the internal pressure of a fuel tank for a long time with comparatively small power consumption after the stop of an engine.
A purge control valve and an atmosphere switching valve of a canister are closed after the stop of the engine to seal the evaporation system, and the power source voltage is supplied to a control circuit (on-vehicle computer) every predetermined time to repeat the processing for detecting the internal pressure of the fuel tank. A maximum value Pamax of the fuel tank internal pressure and a minimum value Pamin of the fuel tank internal pressure are determined during the leakage diagnosing period, and the pressure difference (Pamax-Pamin) between both values is compared with a predetermined leakage determination value after the termination of the leakage diagnosing period, to diagnose the presence or absence of the leakage in the evaporation system. Here, both of the purge control valve and the atmospheric air switch valve are formed by normally-closed solenoid valves, and a relief valve for limiting the maximum pressure of the evaporation system is mounted in parallel with the atmospheric air switching valve.
MAEKAWA YOSHINORI
WAKAHARA KEIJI
MIWA MAKOTO
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