Almost all published work involving injector nozzle cleanliness has been performed by means of keep-clean tests. Another area of interest in the performance of detergents is that of clean-up, which is the ability of a detergent to remove excessive deposits built up in previous service. A study is now reported in which the degree of clean-up is established for a modern commercial detergent used in a low-sulphur diesel fuel. Measurements are presented from a bench engine and also from road vehicles operated under controlled urban driving conditions. Substantial clean-up is demonstrated using detergent treat-rates representative of modern formulation technology, regaining the state of cleanliness that provides optimum performance of the injection equipment. The fouling levels after clean-up are comparable with the results of keep-clean tests, run under identical conditions. Accumulated driving distance and fuel usage to provide this clean-up effect are also discussed. The reductions in fouling are then used to predict a reduction in vehicle exhaust emissions and better fuel economy, by comparison with the results of previous studies.