This workshop service repair manual covers Volvo Penta 230/250/251 DOHC series engines, including related models such as the AQ131, AQ151, and AQ171. These inline 4-cylinder, DOHC engines, with displacements ranging from approximately 2.3 to 2.5 liters, are known for their robust engineering and reliability in marine applications. The manual provides comprehensive information for various production years, detailing engine configurations, performance specifications, fuel delivery systems, and cooling methods, ensuring owners and technicians have access to the necessary data for optimal operation. The purpose of this English-language manual is to provide detailed guidance for the maintenance, repair, and troubleshooting of these Volvo Penta engines and their associated marine drive units. It includes step-by-step instructions, detailed illustrations, diagrams, photos, wiring schematics, and critical notes, cautions, and warnings. The manual is designed to assist owners, mechanics, and technicians in performing scheduled maintenance, diagnosing issues, and completing repairs effectively, ensuring the longevity and performance of the vessel's propulsion system.