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Why does my Jeep Grand Cherokee radiator overflow bubbles?
On a hot day with the A/C running, the radiator overflows and bubbles into the overflow tank after the engine is shutoff. I replaced the bad 195-degree thermostat, which fell apart, with a new 180-degree thermostat. I replaced the hoses and checked the coolant level. On a normal day, the temp gauge reads around 185. On the hot days that it bubbles in the reservoir the temp gauge reads about 210, which is well within the safe zone. What's going on?
Coolant doesn't evaporate in the engine, it can boil, but not evaporate. The reason coolant bubbles like that is because it's boiling. Marc R is right about the hot engine transfering heat to the coolant after it is turned off and raising the coolant temperature even more, but it shouldn't boil yet.
The two things you need to look at to resolve this problem is the radiator cap and the coolant.
The radiator cap is supposed to hold to a certain pressure, usually about 15 lbs. As they get old they can begin to fail. Buy a new radiator cap, they're not that expensive. Check the surfaces where the cap sits and make sure it is clean and free of imperfections (dents, bumps, debris, etc.)
Next, the coolant. Water by itself will boil at about 212 degress fahrenheit in free air. In a closed system like a car's cooling system, it can be higher because of the pressure held by the radiator cap. Anti-freeze additive does 2 things, it helps lower the temperature at which the coolant will freeze, and it raises the temperature at which the coolant will boil. If your coolant hasn't been change for awhile or it doesn't have the correct amount of anti-freeze mixed in, it will boil at too low of a temperature. Best to flush out your system and add fresh water and anti-freeze back in at a 50-50 ratio. On my Jeep I'll put in a gallon of pure anti-freeze and top off with water after a flush.
Make sure to fill your overflow tank with the same solution to the proper level. Run your Jeep a couple of days and check your coolant level in your overflow tank first thing in the mornings when it's cold and add fluid as necessary as the cooling system will take in what it needs at night. You'll need to do this for a few times until the system equalizes.
This should fix you up. Best of luck.