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A simple mathematical model is proposed for the formation of engine deposits and their removal with detergent additive packages. The model is phenomenological and is intended to provide a framework within which the often complex experimental observations can be understood; it does not aspire to be fully predictive or to provide insights into the physics and chemistry of deposit formation. It is applied to inlet-valve deposit formation and removal, and the results demonstrate that two fuels, which might look similar in terms of deposit formation tendency, in a fixed duration engine test, might respond quite differently to the same detergent package. Two additive packages that give identical results in a keep-clean experiment can show quite different clean-up capabilities. A package that has very good keep-clean capability might not show any ability to clean up dirty valves but will do so at higher dose rates.