The Mercury Mariner 150 HP DFI Optimax outboard motor, produced between 1998 and 1999, represents a significant advancement in marine propulsion technology. This 2-stroke Direct Fuel Injection (DFI) engine offers 150 horsepower and a displacement of 2.5 liters, delivering robust performance for a variety of boating applications. Its engineering emphasizes reliability and efficiency, characteristic of the Optimax DFI generation. The motor features a water-cooled system, CDI ignition, and an outboard gearcase with an approximate 1.87:1 gear ratio, designed for optimal power transfer and durability on the water. This comprehensive Service Repair Manual provides detailed guidance for the maintenance and repair of your Mercury Mariner 150 HP DFI Optimax. It covers essential procedures, technical specifications, and troubleshooting steps necessary to keep your engine operating at peak performance. Topics include lubrication requirements, fuel system details, spark plug specifications, and electrical system information, such as alternator output and battery recommendations. The manual is an indispensable resource for owners, technicians, and mechanics seeking to ensure the longevity and proper functioning of this powerful marine engine.