Boiling coolant



When I’m driving on the highway, my heater won’t blow hot air (it’s barely even warm), but as soon as I stop the car, the coolant boils and steam pours out of the hood. All the while, the temperature gauge says that the car is running in its normal range. What is wrong here?


Bad radiator cap, blown head gasket…


How can I tell which one? It looks like coolant has been blowing out of the cap, if that helps.


Blown Head Gasket…

Coolant is full of bubbles.
Radiator stays pressurized even when cold…
Engine appears to boil over but it’s not boiling…
Coolant in recovery tank is never recovered…
With a cooling system pressure tester connected to the radiator, the pressure goes up quickly, without pumping, even when the engine is cold, every time you rev the engine…
A shop uses a “sniffer” to detect combustion gasses in the radiator…


How can I tell which one?

The cap is inexpensive.
Replace it and see if the problem goes away.


I did your test and I think the head gasket is ok. The cap was a little loose and quite a bit of water had boiled off. I tightened it up, added a gallon of water, and drove it around for an hour on and off the highway without boiling. Thanks for the help.


Glad it only appeared to be a radiator cap. You need to get the proper mix of coolant and water, as water alone is not as effective. Being as how it may have been a while since the system was up to pressure, and we don’t know the year, check that your hoses look good and keep an eye on the level.


If the heater is not working properly, you have a separate problem, could have a partially plugged heater core. A cooling system flush could be in order. Also, you air door under the dash could need adjusting.