This service repair manual covers the Mercury Mariner 135 HP DFI Optimax outboard motor, manufactured between 1998 and 1999. As part of the First Generation Optimax DFI series, these engines are characterized by their Direct Fuel Injection (DFI) technology, producing 135 horsepower from a 2.5-liter (153 cu. in.) 2-stroke engine. Engineered for reliability and performance, the Mercury Mariner Optimax series offers advanced fuel efficiency and emissions compliance for its era, featuring a water-cooled system, CDI ignition, and power trim and tilt. This manual provides comprehensive guidance for the maintenance, repair, and troubleshooting of your Mercury Mariner 135 HP DFI Optimax. It details essential procedures, technical specifications, and safety information necessary for optimal engine performance and longevity. Coverage includes engine operation, fuel and ignition systems, cooling, lubrication, electrical components, and the transmission. By following the instructions within this manual, owners and technicians can effectively maintain and service the engine, ensuring its continued reliability on the water.