Yamaha Impeller Housing (WT40X) YJA-01156-30-02. Overview. The Yamaha YJA-01156-30-02 Impeller Housing (WT40X) is a genuine original equipment manufacturer component designed to protect and support the water pump impeller within the engine's cooling system. The impeller housing forms a critical part of the water pump assembly, providing the chamber where the impeller rotates to circulate cooling water throughout the engine.. A properly functioning impeller housing helps maintain efficient water