Yamaha - Water Pump Impeller Cooling System 6AW-44352-00-00. Overview. The impeller is the heart of the outboard cooling system, moving water through the engine to keep temperatures under control. The Yamaha 6AW-44352-00-00 Impeller spins inside the water pump housing, pulling water up from the lower unit and circulating it through the cooling passages. Over time, rubber impellers wear down, lose flexibility, or develop cracks from heat and debris. Replacing the impeller during scheduled mainten