In this paper, an internal combustion engine coolant temperature model has been developed for control and diagnostics development and validation purpose. We propose an approach of using both physical laws and dynamic data to develop the dynamic model of the cooling system. The parameters of the equations for the selected components are collected from vehicle under test and from available literature. The unknown parameters are estimated using a constrained non-linear optimization formulation and dynamic data measured from the test vehicle. The results obtained by the proposed model are closely matching with the actual Engine Coolant Temperature responses for various drive cycles.