The cooling system of an internal combustion engine affects various key parameters, including ignition delay, fuel evaporation, and the compression-expansion process. The geometry of its components impacts both cooling efficiency and the energy demand of the coolant pump. This paper presents CFD simulation results of coolant flow through selected elements of a modified cooling system in a prototype engine. Due to design changes in the power unit, the original coolant outlet manifold required redesign. Three manifold variants were analyzed under two coolant types and two back pressure levels. Flow patterns were evaluated using streamlines and velocity distributions in selected cross-sections. The applied manifold modification significantly increased coolant velocity. In the analyzed cases, the local resistance coefficient rose to values as high as 9. For version mod2, the risk of turbulence was higher with a water-glycol mixture than with water. For version mod1, turbulence sensitivity to coolant type was negligible.