A problem for the diesel engine that remains since its invention is injection nozzle hole fouling. More advanced injection systems and more complex fuels, now also including bio-components, have made the problem more intricate. Zinc and biodiesel have often been accused of being a big part of the problem, but is this really the case?In this study, nozzle fouling experiments were performed on a single cylinder engine. The experiments were divided in three parts, the first part studied the influence of zinc neodecanoate concentration on nozzle hole fouling, the second part studied the effect of neodecanoates of zinc, sodium, calcium, copper, and iron on fuel flow loss and in the last part it was examined how RME concentration in zinc neodecanoate contaminated petroleum diesel affected nozzle hole fouling propensity. After completed experiments, the nozzles were cut open and the deposits were analyzed in SEM and with EDX.The most important conclusion from the single cylinder experiments was that not only zinc neodecanoate can cause nozzle hole deposits, also sodium, calcium, copper and iron salts significantly fouled the nozzle holes. There was also a trend that a higher charge of the metal cation in the carboxylic salt increased the fuel flow loss.The EDX analysis found, in most cases, the metal ion of the contaminant in the nozzle hole deposit, confirming that all neodecanoates were indeed the cause of the rapid nozzle fouling. A general trend was that a higher concentration of the contaminant was found at the entrance of the nozzle hole compared to the exit.From the SEM pictures it was also seen that the carboxylic salts precipitated and formed granulates in the size range 2-4 μm, which is suspiciously close to the porosity of modern common rail fuel filters.