Leakage detecting device for an airtight vessel

A leakage detecting device for an evaporated fuel gas system of an internal combustion engine, comprising:

• a) fuel tank (1),
• b) an evaporated fuel gas adsorbing canister (2) in open communication with an air/vapor space above fuel in the tank,
• c) first control valve means (9, 10) for communicating one end of the canister with the atmosphere,
• d) second control valve means (7, 8) for communicating another, opposite end of the canister with a negative pressure air intake (5) of the engine,
• e) an oxygen sensor (21) in open communication with said fuel tank space, and
• f) an electronic control unit (15) means for:
• 1) closing the first valve means,
• 2) temporarily opening the second valve means to draw a partial vacuum in the fuel tank space and the canister, and thereafter closing the second valve means,
• 3) determining when an increase in the oxygen level in the fuel tank space and the canister over a predetermined time span, as detected by the oxygen sensor, exceeds a predetermined normal value, and in response thereto
• 4) producing an alarm.