So i was looking at the overflow pressure tank in the 05 equinox that i just bought and it looked kinda dingy inside. So i went to go take the cap off cause i wanted to see how dirty the coolant was and kaboom the cap blew off like it had 100 psi behind it. Wel because antifreeze exploded everywhere the check engine light came on probably because coolant got on the coils or spark plugs and of course it was spitting and sputtering. So we drove it for a little bit then decided to take it to the dealership because of the check engine light. Well wouldnt you know on the way there it was driving fine and then as soon as we pull into the dealership the check engine light goes away. So obviously the coolant had evaporated and there was no other problems other then the psi in the pressure tank being way to high. oK SO NOW TO MY REAL PROBLEM! Why is this overflow pressure tank psi so high. Well ok so the dealership did the hydrocarbon test on the cooling system and found that in fact exhaust gases were getting into the coolant system. But there are no symptoms of a blown head gasket. The engine temp stays very steady and there is no big cloud of white smoke behind me and other then the fact of the coolant on the coils the car drives very nice. So i would like anyones opinion on if you really think this car needs a head gasket a week after i bought it. Has anyone else experienced high pressure in this tank. The car did sit for about a half an hour before i opened the tank. Although the coolant that did spray all over me and my fiancee was pretty warm. So does anyone think the pressure was so high just because it was not cool enough? I have heard that the hydrocarbon tests are kinda hear and there. Any help would be appreciated.