A difficulty which exists when applying diagnostic techniques on large-scale diesel engines operating on the field, is that usually it is not possible to obtain measurement data at a wide engine operating range due to a number of constraints. In the present work is investigated the possibility to overcome this practical difficulty originating from the test procedure for engines operating on the field (i.e. marine or stationary applications). The main objective is to examine if a diagnosis procedure provides similar results when applied at various engine operating conditions. For this purpose an existing diagnostic technique, developed by the authors, is applied at different operating conditions on a large-scale two-stroke diesel engine used for power generation in a Greek island. From the diagnosis analysis, several engine operating parameters (i.e. peak firing pressure, cylinder fuel flow, brake power output, injection advance etc.) are estimated at each operating point, as well as the current operational condition of the engine and its subsystems (i.e. compression condition, compressor and turbine condition etc.). From the comparison of the diagnosis results (i.e. operating parameters, engine condition and tuning) at all operating points examined, rather similar results are obtained, regardless of the operating point at which the diagnosis is applied. However, at one operating point, the diagnosis results differ, which as revealed is attributed to load variation during measurement. For this reason, the diagnostic technique has been modified to account for load variation during a measurement. From the present analysis, it is shown that it is possible to estimate engine current condition and tuning from the application of the proposed diagnostic technique at a single operating point, since most of diagnosis results are similar, regardless of the operating point at which diagnosis is applied. This is a serious advantage of the proposed technique, especially for large-scale diesel engines (i.e. marine), where it is not easy to perform measurements at a wide engine operating range, as required.