I had a similar experience with my VOlvo when I passed through the Eisenhower Tunnel on the way into Denver on a 90+ day. It turned out to be a small hose that was not pushed all the way onto the nipple under the small plastic air pump cover on the drivers side front of the engine. It was only about 2/3 of the way on, and under such extreme pressure temperature conditions, it must have leaked a little bit. Take the plastic cover off and check the tube connections. I worked the tube the rest of the way on, and I have not had trouble since. Good luck.