Because of increasing stresses in combustion engines and critical comfort requirements of engine warm-up behavior, FEV has placed a special emphasis on solving cooling system problems. In addition to 3D-CFD calculations and special FEV measurement techniques - such as fiber optical cavitation detection, instationary heat balance measurements during warm-up, etc. - FEV has developed a 1D computer code, known as ‘COOL’, to optimize cooling systems already during the engine design phase or to analyse and eliminate weaknesses in the coolant circuit of existing engines.Beside the algorithm and structure of COOL the paper mainly presents the analysis capabilities of the code. In this connection the emphasis is placed on examples to the current OEMs problem: transient warm-up of DI-diesel engines.The COOL-code is so far a unique CAE tool which exclusively has been applied to projects conducted by FEV. Because of the increasing demand it is planned to commercialize the code in 1998. The paper closes with an outlook to the strategy of the future software development 1) with view to the automotive applier demands.