This case study is aimed to investigate how the injector fouling can influence both the injection system and engine performances. In particular, the presence of deposits in the nozzle of the injector could affect the injection system performance along its life. The investigation was carried out on transparent compression ignition engine equipped with the head of a commercial multi-cylinder engine and second-generation common rail injection system. Two indirect-acting piezoelectric injectors were tested: one new another fouled. Optical engine was fueled with diesel and tests were performed with engine running in continuous mode. Two operating conditions were investigated. The new and fouled piezo injectors were characterized by injection rate profiles. The injection and combustion phases were investigated by optical measurements. Two-color pyrometry was used to analyze the pollutants formation and exhaust emission. Experimental results showed that the fouled injector has slower dynamic response and it injects a smaller amount of fuel during the main event. It has shorter jets penetration and wider spray angle that affect in negative way the mixing formation and the combustion evolution. High temperature regions and high sooting flames are detected for the fouled injector. Therefore, it emits high nitrogen oxides and particulate matter at the engine exhaust. (C) 2013 Elsevier Ltd. All rights reserved.