The present paper reassumes the results of an experimental study focused on the effects of the nozzle injector's coking varying the flow number (FN); the performance and emissions of an automotive Euro5 diesel engine have been analyzed using diesel fuel. As the improvement of the diesel engine performance requires a continuous development of the injection system and in particular of the nozzle design, in the last years the general trend among OEMs is lowering nozzle flow number and, as a consequence, nozzle holes size. The study carried out moves from the consideration that a reduction of the nozzle holes diameter could increase the impact of their coking process.For this purpose, an experimental campaign has been realized, testing the engine in steady state in three partial load operating points, representative of the European homologation driving cycle, and in full load conditions. Three sets of injectors with different flow number (480 cc/30s, 390cc/30s, 300cc/30s) have been used for the experimental campaign; in a first phase new injectors have been used, then, after an accelerated coking procedure of the same injectors, the tests have been repeated to compare the results. The impact of the different FN and coking grade on the engine behaviour has been investigated performing exhaust gas recirculation ratio and lambda sweeps, at partial and full load engine conditions, respectively.The study has evidenced the benefits of the FN reduction on smoke emissions especially at low load conditions. Also as expected, it has been observed a general deterioration of engine performance in the tests carried out after the injectors' coking procedure, that is evidenced by an increase of smoke, CO and HC; moreover the results have shown a greater tendency to suffer the effects of coking for the nozzle with lower FN.