Underhood environment of a passenger vehicle consists of critical components such as heat exchangers, engine, batteries and exhaust system with complex geometries. The exterior styling and the packaging constraints along with the aerodynamic requirements of minimal grill opening areas result in a compact and packed underhood. In such a restricted environment the volume of air flow entering the underhood reduces. The airflow management issues become even more severe in case the underhood environment is located at the rear end of the vehicle, away from the ram air zone available in front of the vehicle, as is the case in the present study. In recent times, a combination of 1D and 3D simulations have gained a high importance to conduct air flow and thermal simulations of vehicle underhood to understand the complex interactions of air flow velocities and temperatures. In the present study, engine cooling performance is studied by evaluating radiator heat dissipation and Top Hose Temperatures (THT) using 1D (KULI) and 3D (FLUENT, RADTHERM) simulation codes. Various design modifications in underhood area are analysed through simulations. These design changes resulted in improved heat dissipation performance of the radiator and better airflow and thermal management of the underhood. Simulation results at steady state operating conditions are compared with vehicle test results and a good agreement observed. This work illustrates the potential use of simulation codes (1D and 3D) to predict underhood thermal environment with acceptable accuracy and to identify areas of improvements for the engine cooling system performance.