Coolant System Mysteriously Pressurizing

[RICK LAKE]

danc30 Dan Call Keith on this. Is the motor all aluminum or iron block/ aluminum heads?? How many miles are on it?? The only idea I have is a head gasket is bleeded down pressure from a cylinder after a heat cycle and some of the pressure is ending up in the coolant system. How much coolant are you leaken? Collect it in a overfill tank and check it how much. I'm thinking about heat cycles of expansion are a possible cause of this. I had this a long time ago and it was a head gasket not sealing. Is the car out in the sun all the time, with or without a car cover on it? This will also cause your problem if the cover can't breathe. Everything will get real hot and give you the same issue. Rick L. Ps I just saw your pictures and you have metal shields under the hood, this will cause the same thing after the car has been out for a run. Put a temp gauge under the hood and check the temps a couple of times after the car has cooled off and you put it away. That metal does reflect heat back at the motor and build up pressure from the car just sitting. The cap will hold pressure to the rating of the cap. ONLY vacuum will suck the coolant back into the system and stop when pressure is equal on both sides. You may have no problem at all. Is the car overheating or running real hot? Does the temp of the coolant go up in the 210-220f and hold there? Rick