This is a 2003 Honda Civic with about 140k on it.
I've had this car checked out by a mechianic and the thermostat was replaced had a pressure test to check for air pockets, nothing. Had a test for a bad head gasket with some sort of chemical test which came back negative. But the car still overheats. I was told by this mechanic that there is a pinhole-sized leak in the head gasket somewhere that leaks into the engine very slowly.
I have to drive about 60mi every day and as I drive at high speeds the car is fine it doesn't overheat. It's only when I am at idle or driving slowly that the gauge begins the rise. On these daily trips I have the heater blowing the entire time because I know it acts like another fan to take heat off the engine. But when I am at a red light all the sudden the air which is on the hot position will turn cold and then the gauge will start to climb. Is this normal for a blown head gasket? or when the coolant gets low. By the time I get to my destination is the coolant is basically gone and it's about the same time I am close to my destination that the air turns from hot to cold and overheats.
Also, this more after my 60mi drive it overheated and I looked under the hood and saw that coolant was sprayed all over the air intake and downwards below of the overflow could the overflow be spitting all the antifreeze out?