As direct injection technology in gasoline engines has become increasingly sophisticated to seek better performance, even a small amount of deposit can have a negative effect on the functionality of injectors. Against such a background, gasoline with effective additives is expected to fix this issue, however, the clean-up and keep-clean processes are not fully understood. In this study, a direct injector fouling test has been developed in order to inspect in more detail injector fouling phenomena in gasoline engines. The test engine used was a 2012 downsized supercharged direct injection spark ignition (DISI) engine equipped with an injector of maximum 15MPa injection pressure. The test fuel consisted of one regular grade gasoline (RG) and three premium grade gasoline with different concentrations of detergent. In the study, it was validated that the developed engine test generates gasoline direct injector deposit (GDID), and SEM-EDX analysis confirmed the presence of GDID inside and on the outer surface of injectors. Different concentrations of detergent showed different GDID formation tendencies under the same conditions. The fitting analysis revealed that low detergent levels are not able to keep the injector surface free of deposit, indicating that a minimum amount of detergent is required to prevent deposit formation on injectors. On the other hand, it was implied that higher concentrations of detergent could accelerate the GDID formation reaction, even though it is generally understood that addition of detergent leads to beneficial effects.