An expansion tank is an integral part of an automotive engine cooling system. The primary function of the expansion tank is to allow the thermal expansion of the coolant. The expansion tank will be referred as hot bottle in this paper. In the System level modeling of the engine internal flow, it is imperative to accurately model and characterize the components in the system. It is often challenging to define the hot bottle accurately with limited parameters in the 1D modeling. Currently it is very difficult to optimize the system by testing. Since testing consumes a lot of time and changes in development stage. If the hot bottle component is not defined properly in the system network, then the system flow balancing cannot be predicted accurately. In this paper, the approach of creating a 1D modeling tool for hot bottle flow prediction is discussed and the simulation results are compared with the physical test data. The variables selected are inflow loss coefficient, outflow loss coefficient for both inlet & outlet branches of the bottle, minimum and maximum surface pressure, initial liquid level and the simplified geometry of the bottle. 1D simulation is carried out using Flowmaster® software. The inputs for the simulation are pressure, temperature and coolant type and output is the mass flow rate through the bottle. Flow rate values observed from the bench test setup were considered to validate the simulation results and the modeling parameters are optimized using Design for six sigma (DFSS) methodology. Hot Bottle flow rate predicted using the 1D modeling correlated well with the test data within 10% error. This methodology can be used for future programs and helps in optimizing the hot bottle flow early in the product development cycle.