This Service Repair Manual covers the Mercury Mariner 30-40 HP (4-Stroke) outboard motor. Engineered for reliability and performance, this water-cooled, inline 3-cylinder engine features Electronic Fuel Injection (EFI) for optimal fuel delivery and a Capacitor Discharge Ignition (CDI) system. Its robust design includes forced lubrication with an oil pump and a Forward, Neutral, Reverse (F-N-R) gear shifting mechanism, making it suitable for a variety of marine applications where dependable power is essential. This comprehensive manual provides detailed, step-by-step instructions supported by hundreds of photos and illustrations, including enlarged insets for intricate details. It covers all aspects of maintenance and repair, from routine servicing such as oil changes and spark plug replacement to in-depth troubleshooting and technical specifications. Safety is paramount, with clear notes, cautions, and warnings integrated throughout to ensure safe operation and maintenance practices, protecting both the user and the environment.