Patent ID: 7367326

Claim:
A failure diagnosis apparatus for diagnosing a failure within an evaporative fuel processing system which includes a fuel tank, a canister having adsorbent for adsorbing evaporative fuel generated in said fuel tank, an air passage connected to said canister for communicating said canister with the atmosphere, a first passage for connecting said canister and said fuel tank, a second passage for connecting said canister and an intake system of an internal combustion engine, and a purge control valve provided in said second passage, said failure diagnosis apparatus comprising: pressure detecting means for detecting a pressure in said evaporative fuel processing system; control means for controlling an opening of said purge control valve by changing a duty ratio of a drive signal which drives said purge control valve; first filtering means for performing a first filtering of the pressure detected by said pressure detecting means, second filtering means for performing a second filtering of the pressure detected by said pressure detecting means, a second passing frequency band of the second filtering being narrower than a first passing frequency band of the first filtering; and flow rate abnormality determining means for determining a flow rate abnormality of a purge gas flowing in said second passage, based on the filtered pressures outputted from said first and second filtering means.