This service repair manual covers the Mercury MerCruiser GM 4 Cylinder 3.0L 3.0LX # 13 marine engine, designed for marine propulsion. This engine features a GM 4-cylinder configuration with a 3.0L (181 CID) displacement and utilizes a Mercarb 2 Barrel fuel delivery system. It is equipped with seawater or closed cooling systems and is built for reliability in marine environments. This manual applies to models with serial numbers OC856451 and above for the MCM 3.0L, and OC868143 and above for the MCM 3.0L 3.0LX. The purpose of this manual is to provide comprehensive, step-by-step written procedures, supported by detailed illustrations and exploded views, for the maintenance, troubleshooting, removal, and installation of the Mercury MerCruiser GM 4 Cylinder 3.0L 3.0LX engine. It covers essential engine systems including the engine mechanical, starting, ignition, charging, instrumentation, electrical kits, wiring diagrams, exhaust manifolds/elbows, and power steering. This resource is intended to assist owners, mechanics, and technicians in effectively servicing and maintaining their marine engines.