Hello everyone!

Yesterday, while giving my car her weekly inspection, I noticed a small coolant leak from the top radiator hose where it attached to the thermostat housing. I tightened the clamp and proceeded to clean up the leak. Since the engine was still hot, I couldn't get a good reading of the coolant level in the reservoir. This morning I checked the level and found the tank to be more than full. I've seen this before about the same time last year. When ambient temperature increases, my car begins to form gas in the cooling system which leads to a build up of pressure (even when stone cold). I've dealt with this issue last year by changing all hoses, thermostat, fan clutch and coolant. In October of last year the radiator was changed along with the coolant. Now, almost 6 months later of flawless operation, my problem is back. I have not yet had a chance to take any temperature readings around the engine to check if perhaps it is exceeding the boiling point of the coolant. My guess is that with our rising ambient temperatures, which get as high as 38 degrees C (100 degrees F), the coolant is on the verge of boiling and forms gas, which then accumulates and overpressures the system.

Has anyone ever dealt with this sort of issue? I can switch to a different coolant if anyone can suggest one that has a higher boiling point. I'm currently using the BMW blue coolant in a 50/50 mix with water. No other additives. Would something like plain water with Redline Water Wetter help? To answer some questions in advance: I've previously checked for headgasket leaks using a tester that detects combustion gases in the coolant. No leak detected. Thermostat is functioning to the best of my knowledge. The car isn't overheating. Fan clutch works...and its on a LOT in the heat. I've determined that gas is being formed based on the fact that when the car is warm and I open the bleed screw, gas escapes first and then I get coolant coming out. When engine is running, I get coolant only...thus the gas also travels around the engine.

Sorry for the long spiel, just trying to provide as much information as possible. Any input is much appreciated!