An evaporated fuel processing apparatus has been widely used, which is configured to: temporarily adsorb evaporated fuel in a canister for preventing the evaporated fuel from flowing to the outside, wherein the evaporated fuel occurs in a fuel tank of a vehicle, and the canister is made of an adsorbent such as activated carbon; and thereafter purge a fuel component from the canister by introduction of fresh air and introduce the fuel component into an intake air line of an internal combustion engine while the internal combustion engine is operating.
A patent document 1 discloses an evaporated fuel processing apparatus which includes a blocking valve in a passage between a fuel tank and a canister, and is configured to adsorb evaporated fuel from the fuel tank into the canister, basically only during fuel filling. This system is configured to maintain the fuel tank in hermetic state by the blocking valve when a vehicle is stationary and no fuel filling is being performed, and thereby more reliably prevent evaporated fuel from flowing to the outside.
The evaporated fuel processing apparatus according to patent document 1 is provided with a diagnostic apparatus for diagnosing whether or not each part is leaking. This diagnostic apparatus includes a negative pressure pump connected to a drain port side of the canister, and performs a leakage check in suitable timing while the vehicle is stationary, wherein the leakage check includes: depressurizing an inside of a system including the fuel tank and the canister; and determining whether or not leakage is present, based on a change in internal pressure of the system at that time.
However, in the conventional diagnostic apparatus, air is sucked by the negative pressure pump from the system via the canister, so that when a large amount of evaporated fuel is adsorbed in the canister, a fuel component may flow from the canister to the outside via the negative pressure pump. Accordingly, this apparatus has a defect that the frequency of situations where the diagnosis is actually possible is low.
Moreover, in the method based on depressurization of the inside of the system, pressure reduction in the fuel tank promotes evaporation of fuel, and thereby increases the amount of evaporated fuel occurring in the fuel tank, so that the evaporated fuel flows into the canister by action of the negative pressure pump, and the adsorbed amount in the canister increases, and the canister thereby approaches a breakthrough (a condition where the amount of evaporated fuel exceeds the capacity of the canister, so that the canister cannot adsorb a further amount of evaporated fuel) undesirably.