My car overheated on the drive in to work today. I'll describe the symptoms...

I've been dealing with a minor coolant leak for about a month now. I top it off once per week, about 1L of fluid. The leak is coming from the upper corner of the radiator (I ordered a new one 2 days ago).

Just recently (2 weeks) I've noticed that when I stop the car, the coolant is boiling. If I open the hood I can see it bubbling up out of the radiator hose into the coolant reservoir. It didn't used to do that.

Also, sometimes the heat doesn't work, especially if the coolant is low. It just blows out cold air. Then I hear a click, and I have hot air again.

Today, nothing was out of the ordinary at first. I just happened to look down at the temp guage and it was edging up to the red. Normally, it sits dead even in the middle. So I turned on the heat and the gauge came down a little. But the heat quit working and the car was blowing cold air for 5 minutes.

After 5 minutes, I hear a sudden thump, then a loud series of thumping noises. I can tell that it's a ton of boiling coolant rushing into the reservoir. I pull the car over and the coolant is boiling like crazy. Car won't start. I let it sit there and cool down.

Once the temp comes down, I try again. Exact same situation. Cold air from the heater, a thump, and a sudden rush of boiling coolant. Then the heat works for a few minutes and then gets cold.

What the heck is wrong? Thermostat? Water pump?

edit: the radiator fan does come on once the car warms up, and when I turn on the AC.

1 Answer 1


tl dr; - Once the radiator is changed, it's probably going to clear up most of the issues.

It could be several issues happening all at once, but let's focus on first things. You're car is most likely boiling over due to the leak in the radiator. The system is not building the pressure it normally would, so will boil sooner than what it would normally. The boiling is exacerbated by there being a lower than normal amount of coolant in the system. This could also be affecting how the heat is working (although, this could be a thermostat issue as well). With coolant being low, this might not be allowing the coolant to circulate very well.

Once you get the radiator put in, ensure the system is fully filled. If you don't do this, you may still encounter some of the same issues you started with. One of the things to help you with this is to put the vehicle on a slope with the nose pointing up. This should allow more of the air in the system to bleed out. Replace whatever fluid is needed after this to fill the system. Ensure you are using a 50/50 mix. If you get full strength anti-freeze, ensure you have distilled water as the other half of the mix (not tap water, as there are minerals and such in the tap water which isn't good for the system). You'll also probably need to check the coolant over the next week to ensure the coolant is good.

If this doesn't clear up your issues, there are two more areas you will need to look at, which are the radiator cap (if it is the pressure relief type), and secondly thermostat. One of these (or all three) should cure your ills, though.

  • The irony of the situation is that I ordered a radiator on Monday. It got delivered to my house while I was sitting on the side of the road today. Going to put it in this weekend, let's hope that resolves the issue. Thanks!
    – AWT
    Commented May 14, 2015 at 17:49
  • 5 year update: it was the radiator :)
    – AWT
    Commented Feb 25, 2021 at 17:06

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