A numerical model that utilizes Computational Fluid Dynamics (CFD) techniques has been developed for the analysis of underhood engine cooling systems of large slow moving vehicles. Several physical models have been developed and incorporated into a CFD code including; a) a model for predicting pressure losses due to screens and grills; b) a model for approximating the forces exerted by the fan on the flow; and c) a model for calculating the heat transfer inside the radiator.The CFD code and physical models have been demonstrated and validated against experimental data. Several three dimensional computational grids that represent various engine enclosures have been created and used to analyze the fluid flow and heat transfer inside the engine enclosure system. The computational results are compared to test data which were obtained for this study. The results are presented in the form of predicted pressure drop and system total flow rate in addition to velocity vectors and temperature distribution inside the engine enclosure.