This paper reports the findings from an experimental investigation of the engine cooling jacket filling process for a medium duty off-highway diesel engine to characterise the physical processes that lead to the occurrence of trapped air. The motivation for the project was to provide knowledge and data to aid the development of a computational design tool capable of predicting the amount and location of trapped air in a cooling circuit following a fill event.To quantify the coolant filling process, a transparent replica of a section of the cylinder head cooling core was manufactured from acrylic to allow the application of optical diagnostic techniques. Experimentation has characterised the coolant filling process through the use of three optical techniques. These include the two established methods of High-Speed Imaging and Particle Image Velocimetry (PIV), as well as a novel approach developed for tracking the liquid-air interface during the fill event. Presented data assesses the influence of several filling process variables on the occurrence of trapped air. These include liquid flow rate, a range of which was selected based on the end-user's coolant filling process, cavity tilt, representing an engine being filled off-level and cavity back pressure, representing downstream cooling system geometries occurring in a real engine. A CFD model of the same event was generated and the validation process used to determine solver accuracy is explained. Conclusions are drawn on the effectiveness of the validation exercise and value of the resulting computational design tool.