A cap for closing a fill tube of a fluid container that includes fewer part and requires just a quarter turn between opened and closed positions. The cap has a lower plate for engagement with a free end of a fill tube and an upper plate that connects to a cover. The upper and lower plates have cooperating pawl and detent portions that form a ratchet member. Rotation of the cap by approximately a quarter turn moves the cap form an opened position to a closed position against the neck of a filler tube.