This workshop service manual covers the Mercury Mariner 150 HP DFI Optimax outboard engine, produced for the 1998 and 1999 model years. This V6, water-cooled engine features Direct Fuel Injection (DFI) and a displacement of 152.6 cubic inches, delivering 150 horsepower. Engineered for reliability and performance, the Mariner series is designed to provide a robust and efficient boating experience. Its advanced DFI technology ensures optimal fuel combustion and power output for a variety of marine applications. The purpose of this manual is to provide comprehensive guidance for the maintenance, repair, and servicing of your Mercury Mariner 150 HP DFI Optimax engine. It details essential procedures, technical specifications, troubleshooting steps, and safety precautions. By following the information contained within, owners and technicians can effectively maintain the engine's optimal performance, ensure longevity, and address potential issues promptly. This resource is an indispensable tool for anyone responsible for the upkeep of this specific Mercury outboard model.