Mercury - Impeller for 8/9.9 Horsepower FourStroke Outboards Cooling System Component Black Marine Elastomer 8M0214945. Overview. There's something reassuring about knowing your outboard is cooling the way it should. You can feel it in the smooth hum of the engine when you throttle up, and in the easy way it settles into idle after a long run. The impeller may be a small part, but it's one of those pieces that gives you quiet confidence every time you head out. With the Mercury Marine Impeller 8