Deposit surface temperature was measured by an infrared technique in a motored engine at the wall temperature level nearly as high as in fired engines. This condition made the infrared measurements free from flame radiation noise. Deposit thermal diffusivity was obtained by measuring the response of a dirty surface thermocouple to a photo-flash. For computational analyses, the deposit was modeled as a solid having the measured thermal diffusivity. The computations used the experimental deposit surface temperature measurements as boundary conditions and established heat conductivity and heat capacity of the deposit. Thermal effects of porous structure of deposits were discussed by solving the fin model which allowed heat exchange between a solid material and a gas inside deposits. In the fin model computation the deposit behaves as if it had larger heat capacity as a single material solid.