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Engine cooling system is crucial to maintain working temperature of vehicles' engine. The two main heat transfer fluids in typical cooling system are coolant and air. Under hood air flow when approaching radiator is highly non-uniform, thus 3D computational fluid dynamics is required to estimate air mass flow rates. While coolant circuit consists of numerous components, 1D thermal-fluid simulation is utilized to study temperature behavior of the coolant. Automotive designers should design a robust engine cooling system which is working well in both normal and severe driving conditions. When vehicles are keyed-off suddenly after some distance of driving, the coolant temperature tend to increase dramatically. This is because soaking heat at engine could not be transferred away timely, as water pump and cooling fan have stop working after keyed-off. In this study, transient coolant temperatures are observed closely with variety patterns of soaking heat. It is suggested to prolong operation of water pump and fan for a short while after keyed-off to reduce transient coolant temperature.