This marine diesel engine service manual covers the Yanmar 2QM20, 2QM20H, 3QM30, and 3QM30H models. These internal combustion engines are designed for marine propulsion and utilize diesel fuel. Yanmar engines are known for their robust engineering and reliability, providing dependable power for maritime applications. This manual is intended to support the continued optimal performance and longevity of these marine diesel engines. The purpose of this factory service manual is to provide comprehensive guidance for the maintenance, repair, and servicing of Yanmar marine diesel engines. It details procedures for general maintenance, basic engine systems, fuel systems, governors, intake and exhaust, lubrication, cooling water, reduction and reversing gears, remote controls, and electrical systems. Additionally, the manual includes operating instructions, disassembly and reassembly guidelines, and inspection procedures, making it an essential resource for owners, mechanics, and technicians.