Mercury - Quicksilver Poppet Valve Diaphragm Cooling System Part Outboards & Inboards 648771. Overview. When your outboard starts running hot or your water pressure fluctuates, it's often the poppet valve diaphragm that needs attention. The Mercury Marine 648771 Quicksilver Poppet Valve Diaphragm is made to keep your cooling system working smoothly by controlling how water moves through the engine. It's built for Mercury and Mariner 75-250HP 2-stroke and 4-stroke outboards, the L6 Verado, and se