Reliable diagnosis of marine diesel engines requires information from multiple diagnostic techniques because individual measurements often describe only selected aspects of engine degradation. This study proposes the Integrated Marine Engine Diagnostic Framework (IMEDF), which organises the diagnostic process by linking degradation mechanisms, physical processes, diagnostic information and maintenance decisions within a unified engineering methodology. The framework was experimentally evaluated using a Sulzer 3AL25/30 medium-speed marine diesel engine operating under healthy and representative faulty conditions. Cylinder pressure and exhaust gas measurements demonstrated that different fault mechanisms produce distinct diagnostic responses and that reliable fault identification requires the combined interpretation of multiple diagnostic methods. The experimental observations also led to the development of the Marine Engine Diagnostic Matrix (MEDM), a practical decision-support tool for selecting diagnostic techniques according to representative fault mechanisms. The matrix provides engineering guidance for selecting and combining diagnostic methods during condition assessment and supports systematic diagnostic planning. IMEDF and MEDM together provide a practical methodology for integrating diagnostic information and supporting maintenance decisions in marine diesel engine diagnostics.