This manual covers the Yanmar 3JH4E, 4JH4AE, 4JH4-TE, and 4JH4-HTE marine diesel engines, part of the JH4 Series. These engines are engineered for reliable marine propulsion, offering robust performance and efficiency for a variety of watercraft. Their design emphasizes durability and operational excellence, making them a trusted choice for demanding marine applications. This comprehensive repair, service, and workshop manual provides detailed guidance for maintaining and repairing your Yanmar marine engine. It covers essential areas including general information, routine maintenance, engine and transmission systems, fuel and lubrication, cooling, electrical systems, and advanced troubleshooting. This manual is an indispensable resource for owners, mechanics, and technicians seeking to ensure optimal performance and longevity of their vessel's power plant.