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A pressure sensor senses a pressure of a fuel vapor system at a fuel vapor passage. An electronic control unit executes a post-depressurization period flow diagnosis to determine whether the fuel vapor passage is clogged based on a behavior of the pressure in the fuel vapor system, which is sensed w

A pressure sensor senses a pressure of a fuel vapor system at a fuel vapor passage. An electronic control unit executes a post-depressurization period flow diagnosis to determine whether the fuel vapor passage is clogged based on a behavior of the pressure in the fuel vapor system, which is sensed with the pressure sensor in a sealed state of the fuel vapor system after introduction of a negative pressure into the fuel vapor system.

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1. A flow diagnosis apparatus for a fuel vapor purge system, which includes a fuel vapor system that has a fuel tank and a fuel vapor passage and purges fuel vapor generated in the fuel tank into an air intake system of an internal combustion engine through the fuel vapor passage, the flow diagnosis

1. A flow diagnosis apparatus for a fuel vapor purge system, which includes a fuel vapor system that has a fuel tank and a fuel vapor passage and purges fuel vapor generated in the fuel tank into an air intake system of an internal combustion engine through the fuel vapor passage, the flow diagnosis apparatus comprising: a pressure sensing unit configured to sense a pressure of the fuel vapor system at the fuel vapor passage; anda flow diagnosis unit configured to execute a post-depressurization period flow diagnosis to determine whether the fuel vapor passage is clogged based on an analysis of the pressure in the fuel vapor system, which is sensed with the pressure sensing unit while the fuel vapor system is in a sealed state after introduction of a negative pressure into the fuel vapor system. 2. The flow diagnosis apparatus according to claim 1, wherein: the flow diagnosis unit determines whether a possibility of the clogging of the fuel vapor passage exists based on the analysis of the pressure in the fuel vapor system, which is sensed by the pressure sensing unit at the time of introducing the negative pressure into the fuel vapor system before the execution of the post-depressurization period flow diagnosis; andwhen the flow diagnosis unit determines that the possibility of the clogging of the fuel vapor passage exists, the flow diagnosis unit executes the post-depressurization period flow diagnosis. 3. The flow diagnosis apparatus according to claim 1, further comprising: a pump that introduces a predetermined pressure into the fuel vapor system upon operation of the pump;a reference pressure sensing arrangement that includes a reference orifice, which has a predetermined diameter;a passage change valve that is operable to change between a passage, which introduce a predetermined pressure from the pump into the reference pressure sensing arrangement upon operation of the pump, and a passage, which introduces a predetermined pressure from the pump into the fuel vapor system upon operation of the pump, wherein the pressure sensing unit, the pump, the reference pressure sensing arrangement and the passage change valve form a pressure introduction sensing device; anda leak diagnosis unit configured to diagnose whether a leak exists in the fuel vapor system, wherein the leak diagnosis unit executes a reference pressure sensing process to sense a reference pressure by changing the passage change valve to introduce the predetermined pressure into the reference pressure sensing arrangement and sensing the reference pressure, which is limited by the reference orifice and also executes a fuel vapor system internal pressure sensing process to sense the pressure in the fuel vapor system by introducing the predetermined pressure into the fuel vapor system, and the leak diagnosis unit determines whether the leak exists in the fuel vapor system by comparing the reference pressure and the pressure in the fuel vapor system. 4. The flow diagnosis apparatus according to claim 1, wherein: the pressure sensing unit senses the pressure of the fuel vapor system at a predetermined sensing location of the fuel vapor passage, which is located between the fuel tank and the air intake system; andthe flow diagnosis unit determines whether the fuel vapor passage is clogged in an area between the predetermined sensing location and the fuel tank based on the analysis of the pressure in the fuel vapor system, which is sensed with the pressure sensing unit at the predetermined sensing location of the fuel vapor passage while the fuel vapor system is in the sealed state. 5. The flow diagnosis apparatus according to claim 1, further comprising: a full level unit configured to determine whether a fuel level in the fuel tank is a full level based on the analysis of the pressure in the fuel vapor system, which is sensed with the pressure sensing unit in a state where the fuel vapor system is sealed upon closing of a purge control valve after the introduction of the negative pressure into the fuel vapor system; anda diagnosis prohibiting unit configured to prohibit the execution of the post-depressurization period flow diagnosis when the full level determining unit determines that the fuel level in the fuel tank is the full level. 6. The flow diagnosis apparatus according to claim 5, further comprising: a fuel consumption determining unit configured to determine whether an amount of consumed fuel, which is drawn out of the fuel tank and is consumed, is equal to or larger than a predetermined amount after the full level determining unit determines that the fuel level in the fuel tank is the full level; anda diagnosis permitting unit configured to permit the execution of the post-depressurization period flow diagnosis when the fuel consumption determining unit determines that the amount of consumed fuel, which is drawn out of the fuel tank and is consumed, is equal to or larger than the predetermined amount.