Mercury - Check Valve Fuel System Component Marine-Grade Construction 8781051. Overview. There are certain small parts on an engine that you don't think about often, yet you feel their presence every time you turn the key. The check valve is one of those quiet helpers. When you've spent long days on the water-running upriver at sunrise or easing across a glassy bay-you learn to appreciate the parts that simply work without demanding attention. This Mercury Marine part gives you exactly that kind