The role and required function of a diesel fuel detergent are discussed and related to the process of nozzle deposit formation. A bench engine test method is described which uses an air-flow technique to measure the fouling produced in IDI nozzles and hence quantify the effectiveness of detergents in the engine test. Data are presented demonstrating the discrimination in detergent performance which may be obtained by applying this method at different operating conditions. Results of ECE 15.04 emissions tests are presented from vehicles having nozzle fouling levels ranging between 1-70%. These emission levels are plotted versus nozzle fouling. From these data, it is concluded that reducing fouling produces a systematic reduction in unburnt hydrocarbon emissions. An optimum fouling level of 15-40% is identified to minimise particulate emissions from this engine.