Glass fiber reinforced polyamide 6.6 has a high service temperature as well as a good chemical resistance. Because of its low raw material price, it is a common construction material for under the hood applications which are in contact with coolants. To keep pace with the developments in the automotive sector, the material has to be improved as well. In the field of polymer synthesis, fiber sizing and additives there is a high potential for further developments. Success will only be achieved if the focus is on the complete system. The influencing coefficients, for example the polyamide 6.6 type, the fiber sizing, the additives and so on will be discussed in this investigation. The main influencing factors will be evaluated. Based on this study the main factor is the fiber sizing, while the influence of different additive masterbatches or polyamide 6.6 types is minor.