This manual covers 230, 250, and 251 DOHC engine families, associated with models AQ131, AQ151, and AQ171. These inline 4-cylinder engines, featuring a DOHC design, offer displacements around 2.3L for the 230 series and 2.5L for the 250/251 series. Power output varies by specific model, with examples like the AQ151 typically producing between 146-150 horsepower. These engines can be configured with either a sterndrive or an inboard marine gearbox, utilizing various sterndrive or marine gearboxes such as the 280 or 290 series. Fuel delivery can be either carbureted or fuel injected, and they employ electronic ignition systems. Cooling is managed via raw water or fresh water systems with a heat exchanger, and thermostats are typically rated at 71°C or 77°C. This comprehensive service repair manual provides detailed information for the repair, maintenance, rebuild, refurbish, and restoration of these engines. It includes diagnostic procedures and wiring diagrams, making it an essential resource for both professional technicians and do-it-yourself mechanics. The manual is presented in English and is fully printable. It covers all aspects necessary to keep your engine operating at peak performance and reliability, ensuring longevity and efficient operation.