A leaking head gasket will cause coolant to gush out and the engine will sputter, spit and misfire. You say the engine runs great. Turning the heater on and off will not cause a steaming head gasket leak. A failing head gasket will leak all the time once it gets to the point of steaming. When you say you turn off the heater do you mean the fan or the interior temperature switch?
Before you run the car again, check the coolant level. Make sure it's full - my guess is that it isn't. Add a little water if it's not too low. Now run the engine and get it to steam again. Open the hood and see where it is coming from. Coolant is leaking somewhere, check around the where the thermostat was replaced. Check especialy around where the upper and lower radiator hoses connect to the engine. Let me know your findings
Tuesday, February 12th, 2008 AT 5:28 PM