Mercury - Impeller Key Cooling System Components Marine Alloy 8M0173458. Overview. There's a quiet confidence that settles in when you know your outboard's cooling system is doing exactly what it should. You feel it when you ease away from the dock, when you push the throttle a little harder against the chop, and when you settle into that familiar steady hum offshore. A small part like the impeller key doesn't get much attention, but when it's right, everything just works - smooth, predictable,