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Current internal combustion engine technology centres around the need for high-energy density products to reduce the size and mass of engines, allowing for improved vehicle performance. This, and the addition of emission reduction ancillaries to meet engine emissions legislation, has led to much greater heat rejection into engine cooling systems since these typically require cooling. It is known that the coolant liquid flow circulates with entrained gas which reduces the performance of the coolant system by de-rating the centrifugal pump and altering the fluid heat transfer and turbulence properties. The source of this gas cannot be remedied hence a device or method is required in the coolant system to remove the entrained gas from the flow. A unique test facility was designed and constructed to replicate an engine cooling system to allow optical diagnostics to be performed on engine cooling system components with full control of coolant flow rate, temperature, pressure, gas flow rate and bubble size. This was used to consider two generic phase separators of the two main types consisting of both a gravity based separator and a centrifugal based separator known as a 'Swirl Pot'. These separators were benchmark tested to provide performance characteristics and these were supplemented by the use of optical diagnostics including high speed imaging and PIV to understand the flow dynamics. This testing found how a typical swirl pot suffered from very low separation performance. The knowledge gained from these separators was used to verify and validate CFD models of the separators showing several deficiencies in the modelling. An investigation into the appropriate modelling conditions and properties of an Extended Life Coolant was performed to improve prediction accuracy and understand realistic property values when in use. This work led to the design of a new separator based on a prior patent design, which was iteratively designed using a combination of CFD predictions and prototyping with testing of these on the test rig. This work increased the performance of the separator across a wider range of conditions, giving valuable design insight and has provided a greater understanding of the conditions within the engine cooling system.