Head or engine block problem

A cracked cylinder head is pretty rare. A cracked block is even more rare. The most common cause of this symptom is a leaking cylinder head gasket. Your mechanic can verify that with a chemical test at the radiator. That involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, the liquid will turn bright yellow.

You can also add a small bottle of dark purple dye to the coolant, then check with a black light after some coolant has been lost. The dye will show up as a bright yellow stain that you can follow back to the source. If a head gasket is leaking, you will find the dye inside the tail pipe. Auto parts stores will have the dye, and those that rent or borrow tools should have a black light.