The influence of elevated temperature, typical for operating fuel injection systems of Diesel engines, on the fatigue strength of such systems is the subject of the current research project “Temperature and High Pressure”. The investigation covered components, whose original durability has been increased by the mechanical procedure of autofrettage (unique mechanical overload) or by the thermochemical process of case hardening. Both methods introduce favourable, fatigue strength increasing compressive residual stresses. Case-hardening additionally increases the local fatigue strength through carburizing the surface layer. No validated information was available concerning the influence of temperature on the fatigue strength of such components with residual stresses for the temperature range of 100 °C to 200 °C. In the project the transient cyclic material behavior of the steel 42CrMo4 (used for autofrettaged components) was investigated as a function of the three test temperatures, room temperature (up to 50 °C), 120 °C, and 180 °C. Material tests were also performed for the steel 18CrNiMo7-6 used for the case-hardened components. The influence of elevated operating temperature on the fatigue strength of autofrettaged and non-autofrettaged components was simulated for cross bore specimens applying both the strip yield model and the finite element method together with the previously identified material behavior. Case hardening was simulated by means of theoretical numerical models and the influence of temperature on the fatigue strength was investigated. Extensive fatigue tests on internal pressure loaded cross bore specimens were carried out for the experimental validation of the theoretical numerical models. Autofrettaged, non-autofrettaged, and case-hardened specimens were tested at the three temperatures mentioned above. The accuracy of all simulation models has been very satisfying. For a simplified endurance evaluation it is recommended to predict the endurance at room temperature and lower the result for the temperature range of 100 °C ≤ T ≤ 180 °C by 10 per cent.