Abstract: A method and apparatus for determining the propensity of fuel materials to leave deposits in compression ignition engines is disclosed. The apparatus consists of a closed chamber fitted with conduits to supply fuel composition, air, and other test fluids to its interior, and arranged to contain or substantially contain a substrate. Substrate is placed in the chamber to provide interaction between a fluid passing through and the substrate. Substrate may be a monolithic machined element or a collection of discrete bodies. The chamber conduits are connected to at least two metering devices, one controlling fuel and one air flow, whereby the substrate is exposed to measured quantities of fuel compositions and air. Heating devices are in thermal communication with the substrate and the source of air provided to the chamber, thus providing a method of controlling the temperature of the substrate and air flowing through the chamber.