The main purpose of this work is the development of a diagnostic procedure for radial turbines installed on internal combustion engine turbochargers. The study is part of a wider research activity regarding the development of diagnostic systems dedicated to cogenerative engine. The proposed procedure is able to formulate a judgment about the turbines efficiency through the real-time evaluation of an effective performance index, only on the basis of pressure measurements across the turbine. Such diagnostic procedure was developed in reference to experimental data gathered in two acquisition campaigns performed before and after a consistent maintenance intervention on engine turbines. The first part of the work approaches turbine failures issues, while in the second one experimental results are presented and analyzed. Finally, the study deals with the definition of some turbine health state indices and their test in reference to the experimental data cited above. A particular index was validated as useful to be implemented in a diagnostic procedure for detection of turbine degradation.