A fundamental vehicle study for a new engine cooling system, the so called NUCLEATE BOILING ENGINE COOLING SYSTEM, has been tested under real and critical conditions in a climate controlled wind-tunnel. Using this cooling system requires a profound knowledge of the cooling circuit and all cooling components such as:condenser, waterpump, liquid/vapor-separator, expansion tank, fan control unit. After preliminary experimental and theoretical studies, two types of cooling circuits were realized and tested in a car: Completely filled nucleate boiling cooling system Partially filled nucleate boiling cooling systemBoth types of circuits were sealed systems. This meant there was no evaporation or loss of liquid into the environment.By using the nucleate boiling cooling system, the heat of the cylinder head was dissipated while the boiling process change of liquid phase to vapor at constant boiling temperature. The considerable heat transfer coefficient during boiling in the engine and condensation in the condenser allows a low liquid flow rate. Furthermore it is possible to get a uniform temperature distribution in the cylinder head. With both types of circuits all the engine cooling requirements can be fulfilled. However, there are some important advantages for the completely filled nucleate boiling cooling circuit.