More than likly you have a pin hole in your heater core and the steam from in the cooling system is the only thing that is able to escape you are likly to be loosing coolant but not at a noticable rate. Monitor your coolant level if it really isnt loosing any coolant than it may be possible the heater has an air pocket in the heater core. I am a shade tree mechanic and have seen this on several older modle cars I really hope it helps.
January, 11, 2011 AT 6:21 PM
That small a leak you can plug with stop leak. The preferred method is to put a 'tee' fitting in you inlet line to the core and add the stop leak there. They sell tees at autoi parts store. And several stop leaks have money back guarantees!