The formation of deposits within injector nozzle holes of common-rail injection fuel systems fitted to modern diesel cars can reduce and disrupt the flow of fuel into the combustion chamber. This disruption in fuel flow results in reduced or less efficient combustion and lower power output. Hence there is sustained interest across the automotive industry in studying these deposits, with the ultimate aim of controlling them.In this study, we describe the use of Scanning Electron Microscopy (SEM) imaging to characterise fuel injector hole deposits at intervals throughout an adaptation of the CEC Direct Injection Common Rail Diesel Engine Nozzle Coking Test, CEC F-98-08 (DW10B test).In addition, a similar adaptation of a previously published Shell vehicle test method  was employed to analyse fuel injector hole deposits from a fleet of Euro 5 vehicles. During both studies, deposits were compared after fouling and after subsequent cleaning using a novel fuel borne detergent.In all cases, the use of fuel borne detergents quickly recovered >75% of power lost during the fouling stage of the tests. SEM images showed that the removal of deposits within the injector nozzle holes was key to the recovery of power and that deposits on the injector nozzle exterior could be misleading in predicting the performance of fuel injectors.