This comprehensive factory service repair manual provides a detailed guide for maintaining and repairing your Mercury Mariner 135 HP DFI Optimax outboard engine for the model years 1998 and 1999. The manual covers a wide range of topics, including:

* **Engine Disassembly and Assembly:** Detailed instructions for taking apart and putting back together your engine, including illustrations and specifications for each component.
* **Troubleshooting and Diagnostics:** Comprehensive guidance on identifying and resolving common engine issues, with troubleshooting flowcharts and diagnostic procedures.
* **Electrical Systems:** Explanations and schematics for the electrical components of your engine, including ignition, charging, and starter systems.
* **Fuel System:** Detailed information on the fuel injection system, including operation, maintenance, and troubleshooting procedures.
* **Cooling Systems:** Instructions for maintaining and repairing your engine's cooling system, ensuring optimal performance and longevity.
* **Lubrication Systems:** Guidance on proper oil selection, maintenance schedules, and procedures for inspecting and replacing oil and filters.
* **Drive Systems:** Information on the lower unit, propeller, and steering systems, including disassembly, assembly, and troubleshooting.

This manual is an invaluable resource for anyone who wants to perform their own repairs and maintenance on their Mercury Mariner 135 HP DFI Optimax outboard engine.

The Mercury Mariner 135 HP DFI Optimax, produced from 1998 to 1999, marked a significant step forward in outboard technology. This engine, designed for recreational boating, incorporated Mercury's innovative Direct Fuel Injection (DFI) system, which revolutionized the way outboard engines delivered power and efficiency. The DFI system, utilizing a high-pressure fuel pump and electronic controls, allowed for precise fuel metering and combustion, resulting in greater power output, smoother operation, and reduced emissions compared to traditional carburetor-equipped outboards. The 135 HP Optimax was renowned for its smooth running characteristics, thanks to the DFI system's ability to deliver a precise fuel/air mixture to each cylinder at all operating conditions. Its lightweight design and powerful performance made it a popular choice for a wide range of boat types, from fishing boats to runabouts.

The 1998-1999 Mercury Mariner 135 HP DFI Optimax was a technologically advanced outboard engine that offered several advantages over its predecessors. The DFI system not only improved fuel efficiency and reduced emissions but also allowed for easier starting and reduced maintenance requirements. Moreover, the engine's compact design and lightweight construction made it easy to install and maneuver, enhancing its appeal to boat owners. The 135 HP Optimax was built with durability in mind, featuring a robust crankshaft and high-quality components, ensuring reliable performance for many years to come. While production of the DFI Optimax line ceased in 2008, these engines continue to be highly sought-after by boaters today thanks to their outstanding performance, efficiency, and reliability.