This manual covers the Yanmar 3YM30, 3YM20, and 2YM15 marine diesel engines. These engines are designed for reliable performance in marine applications, offering a range of power outputs from 15 to 30 horsepower. Engineered for durability and efficiency, Yanmar engines are known for their robust construction and dependable operation, making them a trusted choice for various marine vessels. Their compact design and advanced features, such as direct injection and naturally aspirated aspiration, ensure optimal performance and fuel economy. The purpose of this service repair manual is to provide comprehensive guidance for the maintenance and repair of these Yanmar marine diesel engines. It details essential procedures, including general inspection, adjustments, troubleshooting, disassembly, and reassembly. The manual covers critical systems such as the fuel injection, intake and exhaust, cooling water, lubrication, and electrical systems, as well as reduction and reversing gears. This resource is intended to assist owners, mechanics, and technicians in keeping their Yanmar engines in optimal working condition, ensuring longevity and peak performance.