The most likely suspect is a leaking cylinder head gasket. You can add a small bottle of dark purple dye to the coolant, then search a day later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. Check inside the tail pipe. If you find the dye there, a cylinder head gasket is leaking.
Your mechanic can also do a chemical test at the radiator. That involves drawing air 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.
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Monday, February 23rd, 2015 AT 1:19 PM