Patent ID: 7290535

Claim:
A leak diagnostic apparatus for an internal combustion engine, the apparatus comprising: a canister having an air inlet opening, the canister temporarily absorbing fuel vapor produced in a fuel tank; a fuel vapor passage extending from the fuel tank to the canister; a purge passage extending from the canister to an intake system of the internal combustion engine; a purge valve for opening and closing the purge passage; a cut valve for opening and closing the air inlet opening; and a leak diagnostic control unit for diagnosing a fuel vapor leak in a purge line extending from the fuel tank to the purge valve through the canister, the leak diagnostic control unit operable to: detect a pressure in the purge line; measure a first pressure change rate of the purge line after a predetermined negative pressure is applied to the purge line and the purge valve and cut valve are closed, sealing the purge line at the predetermined negative pressure; measure a second pressure change rate of the purge line after measuring the first pressure change rate, opening the cut valve to expose the purge line to atmospheric conditions, passing a predetermined lag time and closing the purge valve and the cut valve; determine a leak degree based on a difference between the first pressure change rate and the second pressure change rate; estimate a fuel vapor amount wherein the fuel vapor amount is an amount of fuel vapor produced during a time the first pressure change rate is being measured; and shorten the predetermined lag time as the fuel vapor amount increases.