This Service Repair Manual covers the Mercury 90 US / 100 EU Four Stroke EFI outboard engines. These engines are designed with advanced EFI technology for efficient fuel delivery and reliable performance. The 90 US model offers 90 horsepower, while the 100 EU model provides 100 horsepower, both utilizing a gasoline-powered, four-stroke EFI system with electronic ignition and an alternator charging system. Engineered for durability and optimal operation, these Mercury outboards are built to meet the demands of various marine applications, ensuring a dependable boating experience. This comprehensive manual provides detailed information essential for the maintenance, repair, and servicing of your Mercury outboard engine. It includes step-by-step instructions, detailed diagrams, illustrations, and photographs to guide you through all procedures. Topics covered range from routine scheduled maintenance, including flushing procedures for the cooling system and anode inspection for corrosion control, to specific maintenance tasks such as engine oil changes, power trim fluid checks, and gear housing lubrication. Technical specifications, troubleshooting guides, and propeller information charts are also included, empowering owners and technicians to keep the engine in peak operating condition.