The source of an external coolant leak can also he found by a thorough visual inspection or the use of a dva penetrant. Visually, the point of the leak may be wet om have a light gray color, the result of the coolant evaporating at that point. Another common way to identify the source of a leak is to use a dye penetrant and a black light The dye is poured into the cooling system and the engine is run until it reaches operating temperature. With the engine turned off, the engine and cooling system are inspected with the black light. Where the dyed coolant leaks, a bright or fluorescent green color will be seen.