The paper report on the state of affairs so far in the scope of internal diesel injection deposits (IDID) issue affecting modern diesel high pressure common rail (HPCR) systems. Different types of deposits have been described and the factors supporting their creation. These deposits have been extensively studied to understand their formation mechanisms and composition. To this end an engine test procedure was developed to assess the deposit forming tendencies of various types. Findings of two long time dynamometer engine tests in which the course of the IDID formation and HPCR performance impairing was followed by measurement different engine parameters have been reported. Performance evaluation of HPCR injectors before and after tests have been presented also.