The changes of flow field and temperature field of underhood before and after the closure of the back door opening of a bus engine cabin were studied by the numerical simulations and experiments. The results indicated that after the closure of the back door opening, there are no notable changes of the flow field inside the engine cabin. In addition, the closure does not lead to the decrease of the cooling capacity of the cooling system. However, the temperatures near the former opening of the back door and the clutch increased slightly. Nevertheless, the closure of the opening of the back door cannot cause adverse impact to the performance of the bus.