This manual covers the Yanmar 4BY-150, 4BY-180, 6BY-220, and 6BY-260 marine diesel engines. These engines are engineered for reliable marine propulsion, delivering consistent performance across their respective horsepower ranges. Their robust construction and advanced diesel technology ensure efficiency and durability for a wide array of marine applications, making them a trusted choice for boat owners and operators. The purpose of this comprehensive document is to provide essential information for the maintenance, repair, and operation of your Yanmar marine engine. It includes detailed procedures for routine maintenance, engine removal and installation, and in-depth troubleshooting for various systems such as fuel, lubrication, cooling, and electrical. This manual serves as a vital resource for owners, technicians, and mechanics, enabling them to effectively service and maintain their engines for optimal performance and longevity.