The paper deals with non-dismountable diagnostic of the power of a turbocharged Diesel engine. For the purposes of diagnostic the transient operation mode of the engine was described by an extended mathematic model that takes into account the influence of suction volumes and exhaust pipes. The results obtained by this improved model were similar to the values measured by an experiment. This model produces relatively more trustworthy information regarding the influence of some typical defects on the chosen diagnostic parameters. Finally, the improvement makes possible to more accurately identify them, on the basis of changes of selected and measured diagnostic quantities the defect that has caused these changes.