I have a 2002 Honda Civic EX with app. 165,000 miles. For about 9 months I have been losing coolant. I don't lose enough coolant for the engine to overheat but enough for the heater to work off and on. It seems that the coolant level gets to one level and stays there. This was not a problem in the summer as I didn't need the heat. On occasions when I check the coolant I notice that the overflow tank is completly full. When I add coolant, whether it be after 1 day or 3 weeks, it always takes between 12 - 18 oz.. About 5 months ago, I had a mechanic check for leaks but did not find any. Any thoughts?