This manual covers Volvo Penta marine engines, specifically models 230, 250, 251 DOHC, AQ131, AQ151, and AQ171. These engines are engineered for reliable marine propulsion, reflecting Volvo Penta's commitment to quality and performance in demanding aquatic environments. This comprehensive guide is designed to assist users in understanding and operating their marine propulsion systems effectively. The purpose of this manual is to provide detailed information for the maintenance, repair, and troubleshooting of your Volvo Penta marine engine. It encompasses a wide range of topics, including general information, routine maintenance schedules, engine removal and installation procedures, fuel and lubrication systems, cooling systems, engine specifications, transmission, drive systems, steering, electrical systems, and advanced troubleshooting techniques. This resource is invaluable for owners, technicians, and mechanics seeking to ensure optimal performance and longevity of their marine engines.