This manual covers the Yanmar 2QM20F, 2QM20Y, 3QM30F, and 3QM30Y diesel engines. These engines are engineered for reliability and performance, designed for marine inboard applications, as well as general equipment and small engine use. Yanmar's commitment to quality ensures these diesel engines provide dependable power for a variety of demanding tasks. This comprehensive manual serves as a vital resource for owners, technicians, and mechanics. It provides detailed information on operation, maintenance, and repair procedures. Key areas covered include general information, routine maintenance, engine removal and installation, fuel systems, lubrication and cooling systems, electrical systems, and advanced troubleshooting. By following the guidelines within this manual, users can effectively maintain and service their Yanmar diesel engines, ensuring optimal performance and longevity.