This operation manual covers the Yanmar 2QM20 and 3QM30 (F) (Y) marine diesel engines, designed for marine propulsion. These engines are engineered for reliability and performance in demanding marine environments. This manual provides essential information for the proper operation and upkeep of your Yanmar engine, ensuring its longevity and efficient function. The purpose of this English-language manual is to guide owners and technicians through the operation, maintenance, and troubleshooting of the Yanmar 2QM20 and 3QM30 (F) (Y) engines. It details procedures for starting, stopping, and checks during operation, as well as preparation for use and air venting. Additionally, it includes information on periodical checks, parts identification, and the fresh water cooling system, all crucial for maintaining optimal engine performance and safety.