The paper presents algorithms for marine diesel engines’ working process monitoring designed by analyzing data obtained with IMES pressure sensors. Main features and properties of IMES sensors, used for 2- and 4-stroke modern marine diesel engines, are presented here. The pressure-time diagrams were taken as a basis for creating algorithms for marine diesel engines’ working process monitoring. The task of preliminary calculation of top dead center position (TDC) of a cylinder was solved using polytropic compression equation. TDC position was calculated using the equation P' = 0. Then we determine the algorithm for calculating the average indicator pressure and the indicator power of the cylinder. The CFD methods show how indicator valve channel influences the results of marine diesel engines’ working process monitoring. With the example of compression curves in a cylinder, we show amplitude and phase distortions introduced into pressure measurements by the indicator valve channel. Such research work is important and actual in time due to the use of marine diesel portable performance analysis systems.