Patent ID: 7331335

Claim:
A fuel vapor treatment system for an internal combustion engine, comprising: a canister that is connected to a fuel tank through a vapor introduction passage and has an adsorbent for temporarily adsorbing fuel vapor, the fuel vapor being produced in the fuel tank and being introduced into the canister through the fuel vapor introduction passage; a concentration measuring means that measures a fuel vapor concentration in an air-fuel mixture when the fuel vapor is desorbed from the adsorbent; and a flow rate control means that is provided in a purge passage and controls a flow rate of the air-fuel mixture containing the fuel vapor purged into the intake pipe on the basis of the fuel vapor concentration, the purge passage connecting the canister and an intake pipe of the internal combustion engine, wherein the concentration measuring means includes: a measurement passage provided with a restriction; a gas flow producing means for producing a gas flow in the measurement passage; a pressure measuring means for measuring pressure produced by the restriction when the gas flow producing means produces the gas flow; a measurement passage switching means for switching the measurement passage between a first measuring state in which the measurement passage is opened to the atmosphere so that air flows through the measurement passage, and a second measuring state in which the measurement passage communicates with the canister so that the air-fuel mixture containing the fuel vapor flows through the measurement passage; a fuel vapor concentration computing means for computing concentration of the fuel vapor on the basis of a first pressure measured by the pressure measuring means in the first measuring state and a second pressure measured by the pressure measuring means in the second measuring state; and a malfunction determining means that compares a behavior of change in the first pressure with a behavior of change in the second pressure, and determines that the measurement passage switching means has a malfunction when the behaviors of change in the first and second pressures are substantially identical to each other.