Measurement of pressure in the pre-chamber and in the main chamber of an indirect injection Diesel engine provide useful information for the interpretation of the combustion process. However, this is not easy because pressure signals are affected by a number of simultaneous processes such as compression, heat transfer and heat release, and mass exchange between chambers. Diagnostic models try to separate these effects by providing heat release laws which describe with detail the combustion process timing. The whole diagnostic procedure, which is simple and accurate in the case of DI Diesel engines, becomes more complex in IDI ones, and requires much care in calibration, filtering and adjustment because most of the information depends on differences between pressure in the two chambers, whose absolute values are only slightly higher than the accuracy of the pressure measurements. In this work a methodology for the experimental diagnostic is described. The methodology covers the engine testing in both motored and firing conditions, the pressure signal acquisition and processing, the determination of the engine dynamic characteristics, the adjustment of a diagnostic mathematical model, and the final interpretation of the pressure signals through the diagnostic model. Finally, some examples of application are presented.