In order to reduce the cost of engine cooling systems in particular the turbo Diesel engines with charge air coolers, we want to understand the relationship between oil sump temperatures and engine coolant temperatures and their impact on one another. Several cars have been tested in the climatic wind tunnel. The following are the cooling specifications: hill climbing with 12% grade with or without a trailer at a 20°C ambient max. speed at 35°C ambient. The main results of these studies were: a great variation of oil sump temperature versus coolant temperature a small variation of coolant temperature versus oil sump temperature a very small variation of heat flux in the oil, in the coolant and the output of engine versus oil sump and coolant temperature.