This workshop service manual covers the Volvo Penta 230/250/251 DOHC Series marine engines, including variants such as the AQ131, AQ151, and AQ171. These inline 4-cylinder DOHC engines, typically featuring fuel injection and marine cooling systems, were designed for reliable performance in various marine applications. The manual provides comprehensive information for these engines, which offer displacements around 2.3L to 2.5L and power outputs ranging from approximately 130 HP to 170 HP, depending on the specific model. Engineered for durability and efficiency, these Volvo Penta powerplants are known for their robust construction and suitability for demanding marine environments. The purpose of this manual is to serve as an authoritative resource for the maintenance, repair, and troubleshooting of the specified Volvo Penta engines. It includes detailed step-by-step repair instructions, maintenance charts, troubleshooting guidance, overhaul procedures, and wiring diagrams. This extensive documentation is designed to assist both professional technicians and DIY mechanics in effectively servicing these engines, ensuring optimal performance and longevity. The manual covers various transmission and gearbox types commonly paired with these engines, providing essential technical specifications and operational insights for comprehensive engine care.