Exhaust Gas Recirculation (EGR) is a consolidated in-cylinder technique in the automotive industry to decrease the Nitrogen Oxide (NOx) emissions of both diesel and gasoline engines. In order to comply with more stringent NOx limits the recirculating technology has evolved to enable higher EGR flow rates and accommodate more advanced control strategies. The increase in the cooling load demands the utilization of compact coolers with high thermal efficiency and the minimum pressure drop in both gas and coolant sides. In addition to the performance requirements, the design of the EGR cooler has a severe packaging constraint, is affected by the pulsating flow and fouling in the gas side and has to withstand high thermal stresses due to the large temperature gradients between the hot exhaust gases and the coolant.In the development phase of EGR systems is common practice to make the simplifying assumption of steady state boundary conditions to evaluate by experimental and CFD models the performance of the cooler and obtain maps of temperatures to carry out structural analysis. However the location of the EGR system in a highly pulsating environment makes the cooler performance and thermal maps in real engine condition quite different from the predicted and tested with steady boundary conditions.In this paper, in order to quantify the pulsating effects in a HP EGR system, a whole 1D model of a 2 liter, four-cylinder, common rail and turbocharged diesel engine was built in the engine simulation software GT-Power and was dynamically coupled with the CFD model of the EGR system created in Fluent. The transient results of the coupled simulation were compared and discussed with the results obtained in the corresponding steady CFD simulation with the same time averaged mass flow rate and temperature.