The aim of this study is to investigate how the fouling that injectors undergo after several operating hours on a vehicle can affect the injection and combustion phases. The impact of the injector fouling on the pollutant formation has been also investigated. Moreover, the effects of the injector cleaning by deposits through the top quality diesel fuel commercialized by eni that is FAME free and contains multi performance additives have been investigated. The experimentation has been carried out on transparent compression ignition engine. It is a single cylinder equipped with a Euro 5 multi-cylinder head and a second-generation common rail injection system. Three indirect-acting piezoelectric injectors have been tested. The first one has been fouled with European commercial diesel fuel through the CEC DW10 injector-coking test. The second one has been fouled in the same way and, then, it has been cleaned with eni top quality diesel fuel. This fuel has fed the third injector too. Two operating conditions have been investigated; they represent the condition at 1500 rpm and 2 bar of Brake Mean Effective Pressure (BMEP) and at 2000 rpm and 5 bar of BMEP, respectively. Tests have been performed with European commercial diesel fuel. Injection rate profiles of the three injectors have been detected. Steady-state measurements of exhaust emissions have been realized by commercial instruments. Non intrusive 2-D digital imaging has been performed to investigate both the injection and the combustion evolution. It has been found that the injector behavior in terms of spray penetration, combustion efficiency and pollutant formation is influenced in a negative way by the deposits presence. Moreover, it has been observed that the injector cleaning results in a performance improvement and a pollutant emissions reduction.