Cooling of low-drag cars can be very sensitive to front-end air management in hot environments. An iterative simulator of a car-engine-radiator system is presented which predicts the radiator temperature at idle after a heat soak. The procedure uses information gained under laboratory conditions at lower than boiling temperatures which is then used for thermal stability predictions. An estimate of the stored energy in the engine-bay at high temperature is needed.The procedure offers a reasonably simple way of computer prediction of the coolant temperature of a radiator system and assessing the sensitivity of the various parameters. It also offers the potential for less testing-time in the environmental wind tunnel.The analytical simulator appears to behave correctly in response to known state changes. Other than simulating one condition in a car, its ability to satisfactorily predict other conditions for other cars has not been established.