This manual covers the Yanmar 2QM20 and 3QM30 marine diesel engines, designed for reliable marine propulsion. These internal combustion engines represent robust engineering, built for demanding maritime environments. Their construction prioritizes durability and efficient operation, making them a trusted choice for boat owners and operators seeking dependable performance on the water. The purpose of this English-language service repair manual is to provide comprehensive guidance for the maintenance and repair of your Yanmar engine. It details essential procedures for the general engine, fuel system, governor, intake and exhaust systems, lubrication, cooling, and electrical systems. Additionally, it covers the chain-overdrive hand-operated system, reduction and reversing gear, remote control system, installation, operating instructions, disassembly, reassembly, and inspection and servicing, ensuring your engine remains in optimal condition.