This service repair manual covers a range of Yanmar Marine Diesel Engines, including models 2QM20, 2QM20H, 2QM20Y, 2QM20F, 3QM30, 3QM30H, 3QM30Y, and 3QM30F. These diesel engines are engineered for reliable performance in marine applications. This manual provides comprehensive information to ensure the proper maintenance and repair of your Yanmar marine diesel engine, facilitating its continued operational efficiency. The purpose of this manual is to offer detailed guidance for the repair and servicing of these Yanmar marine diesel engines. It includes easy-to-read text sections, high-resolution diagrams, and step-by-step instructions covering essential systems such as the lubrication, cooling water, fuel, and electrical systems, as well as the reduction and reversing gear. Suited for both do-it-yourself owners and experienced mechanics, this manual is an indispensable resource for maintaining the integrity and functionality of your engine.