In Gasoline Direct Injection engines, the direct exposure of the injector to the flame can cause combustion products to accumulate on the nozzle, resulting in an adverse effect on emissions and the lifetime of this component. This research is studying the impact of injector fouling on particulate emissions and the associated spray pattern changes and how both can be reversed by utilising a fuel additive. For this purpose multi-hole injectors were deliberately fouled in four-cylinder test engines with different types of fuel. Recorded particulate numbers (PN) increased two orders of magnitude in some cases, during a four hour cycle. The drift could be reversed by switching to a blend that contained a detergent additive. In addition, it was possible to completely avoid any PN increase, when the detergent fuel was used from the beginning. The combustion deposits on the injector nozzles were analysed with a microscope and their extent followed the same trends observed previously. The beneficial effect of the detergent additive was also present here. Based on these results a selection of injectors was installed in a laboratory injection chamber and the spray patterns were investigated with a high speed camera. Injectors corresponding to the largest PN drift also produced the thinnest spray jets with the deepest penetration. These factors amplify the risk of wall wetting and provide an explanation for the increase of PN. The positive effect of the detergent was also reflected in the spray pattern analysis, which further illustrates possible benefits of such fuel additives.