A quick, convenient, objective and sensitive engine test to monitor the activity of a gasoline detergent additive package in the inlet system has been developed. The test is based on an optical technique which detects, by monitoring atomic emission, the increase in the movement, caused by the additive package, of an appropriate marker from the inlet manifold/port into the combustion chamber. The test is not sensitive to deposit formation tendencies of the fuel and additive package components and hence cannot be used to assess the cleanliness performance of unknown gasolines or of additive packages whose components themselves produce deposits. Since it is developed in a carburetter engine, the results are relevant to carburetter engines. The results from the test are repeatable and correlate very well with results from a variety of more expensive engine and road tests. They are also in line with existing knowledge about detergent packages reported in the literature. The study highlights the importance of the carrier fluid in the additive package for carburetter engines.