This workshop service manual covers the Mercury Mariner 135 HP DFI Optimax outboard engine, produced for the 1998 and 1999 model years. As part of the Early Optimax Generation, this engine features a 6-cylinder, 2.5 Liter (153 cubic inches) Direct Fuel Injection (DFI) Optimax configuration, delivering 135 HP at 5000-5800 RPM. Engineered for reliability and performance, the Mercury Mariner is designed for marine applications, utilizing an outboard lower unit with a 1.87:1 gear ratio for both forward and reverse. The purpose of this comprehensive manual is to provide detailed guidance for the repair and overhaul of your Mercury Mariner outboard. It includes essential information on maintenance procedures, technical specifications, troubleshooting, and safety guidelines, ensuring optimal performance and longevity. This resource is intended for owners, mechanics, and technicians who are fully conversant with marine engine repair, offering the necessary knowledge to maintain and service the engine effectively, including specific recommendations for lubrication and fuel requirements.