Yamaha Water Pump Impeller YJA-01212-27-01. Overview. The Yamaha YJA-01212-27-01 Water Pump Impeller is a genuine original equipment manufacturer component designed to circulate cooling water through the engine and maintain proper operating temperature. The impeller is a critical part of the water pump system, using flexible blades to draw water from the lower unit and push it through the engine's cooling passages.. A properly functioning impeller ensures consistent water circulation while the e