Hi. I have a Pontiac Grand Am 2001. A couple months ago it started to show the temperature gauge going all the way up and after a few minutes going back to normal. I checked the coolant and there was no coolant in the reservoir tank, so I added some. 2 days later... no coolant again. Took it to a repair shop, the guy did a pressure test to see if there was any leak and did not find anything. He checked the oil and it is not mixed with antefreeze. One other thing that the car is doing is that sometimes when I turn the heater on it blows cold air... I am very frustrated, don't know what to do and don't want to spend too much money just on "guessings". Does anybody have an idea of what could be wrong? Please HELP. thanks