When traveling slowly the engine temperature lights move to the red mark. When opening the heater the temperature drops for a while but moves up again. I had the radiator tested at Silverton and they suspect that there is exhaust fumes in the water. Is this an clear indication of a blown to gasket? The pressure inside the radiator remains constant. Do I need to have the top gasket replaced or can I use something like gasket magic liquid added to the radiator to seal the leak if any?. Thanks