This workshop service manual covers Volvo Penta marine engines, specifically models 230, 250, 251, AQ131, AQ151, and AQ171. These 4-cylinder, 4-stroke overhead cam engines are designed for marine propulsion. The manual provides comprehensive information for maintaining and repairing these reliable engines, ensuring optimal performance for their intended use. This manual is a detailed resource for technicians and owners alike, offering in-depth coverage of safety precautions, general information, and specific repair instructions. It includes troubleshooting schemes for fuel and ignition systems, overhaul data for various engine components such as the cylinder head, cooling system, and crank assembly, and procedures for tasks like toothed belt installation. With detailed exploded views, pictures, and diagrams, this manual serves as an essential guide for maintaining the integrity and functionality of your Volvo Penta marine engine.