Mercury - Fuel Tank Diurnal Cap with In-Tank Retainer Fuel System 36-8M0064067. Overview. Portable fuel tanks take a beating-sun, movement, pressure changes-and the cap is often the first thing to quit on you. When it stops sealing right, you get fuel smell, pressure issues, or a tank that bulges in the sun. This Mercury diurnal cap is the correct replacement for the newer 3.2- and 6.6-gallon tanks. It threads on cleanly, vents the way these tanks are meant to, and uses an in-tank retainer so th