This paper presents an experimental technique for locating engine cooling system hot spots by using a Magnesium Borate water solution for the engine coolant. In doing so, Magnesium Borate is deposited wherever local boiling occurs thus indicating the high temperature areas.This technique was applied to a large V-8 engine and provided visual results of high temperature areas in the water jacket. Color photographs show the results, which correlate with measured metal temperatures and measured coolant flow velocities. The extension of the technique to include estimation of metal temperatures results from additional test data reported.The procedure is useful for locating unsuspected hot-spots in any passage in the engine cooling system. The severity of the temperature may be determined by knowing the Boiling Point of the solution.