This workshop manual covers Volvo Penta marine internal combustion engines, specifically models 230, 250, 251DOHC, AQ131, AQ151, and AQ171. These engines are known for their robust engineering and reliability, designed for demanding marine applications. The manual provides comprehensive information for maintaining and repairing these units, which often feature a 4-cylinder configuration and DOHC valve train for enhanced performance, particularly in the 251DOHC and AQ171 variants. This English-language workshop manual is designed to be an indispensable resource for owners, technicians, and mechanics. It details essential procedures, including safety precautions, general information, repair instructions, and troubleshooting schemes. Key areas covered include the electrical, fuel, ignition, cooling, and valve systems, as well as cylinder head and block overhauls, toothed belt installation, and oil pump and cooler maintenance. The manual aims to facilitate efficient and accurate upkeep, ensuring the longevity and optimal performance of your Volvo Penta engine.