The engine cooling system for a typical class 3 pickup truck with a medium duty diesel engine was modeled with a commercial code, GT-Cool in order to explore the benefit of controllable electric pump on the cooling performance and the fuel economy. As the first step, the cooling system model with a conventional mechanical coolant pump was validated with experimental data. After the model validation, the mechanical pump sub-model was replaced with the electric pump submodel and then the potential benefit of the electric pump on fuel economy was investigated with the simulation. Based on coolant flow analysis the modified thermostat hysteresis was proposed to reduce the recirculating flow and electric pump effort, thus enabling assessment of the full power saving potential. It was also demonstrated that the radiator size could be reduced without any cooling performance penalty by replacing mechanical pump with the electric pump and decoupling of the pump speed from engine speed. The predicted results indicate that the cooling system with the electric pump can dramatically reduce the pump power consumption during the FTP 74 driving schedule and that radiator can be down-sized by more than 27% of the original size under grade load condition.