This Service Repair Manual covers the Mercury Mariner DFI Optimax outboard motor, specifically for models produced between 1998 and 1999. These engines are known for their Direct Fuel Injection (DFI) technology, offering efficient power delivery. Available in 135 HP and 150 HP variants, the 150 HP model features a 150 cubic inch (2.45 Liter) displacement. The Mariner DFI Optimax is engineered for reliable performance on the water, utilizing a water-cooled system and an outboard gearcase with Forward, Neutral, and Reverse gears. Shaft length options typically include Standard and Long configurations. This comprehensive manual provides essential information for the maintenance and repair of your Mercury Mariner DFI Optimax outboard. It details critical procedures, including recommended servicing with Mercury Optimax DFI 2-Stroke Oil, and outlines technical specifications for various components. Owners, technicians, and mechanics will find guidance on troubleshooting, system diagnostics, and proper upkeep to ensure optimal engine function and longevity. Adhering to the procedures and specifications within this manual is key to maintaining the integrity and performance of your marine engine.