Mercury - Fuel System Diaphragm TwoStroke & Diesel Series Marine Composite 8M6002988. Overview. When you spend enough time on the water, you start to appreciate the quiet heroes inside your engine-the parts you never see but depend on every time you push the throttle forward. The Mercury 8M6002988 Diaphragm is one of those unsung essentials. It keeps fuel pressure steady, the combustion cycle balanced, and the whole engine feeling smooth and predictable. That reliability becomes part of the rhyt