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Yesterday I was finishing a drive of about 80 miles when my heater quit blowing warm air (it's currently winter here). I noticed the temp gauge was at the red mark, so I pulled off the highway and stopped at a light. The engine immediately shut off when I stopped. I smelled antifreeze and the hood was steaming. I waited for it to cool down a little, then drove it the two blocks home.

There I noticed the coolant reservoir cap was popped open, there was hardly any coolant in the reservoir, but there was coolant all over the engine. I've been reading up on this and it sounds like it could be a few issues, one common one being a head gasket issue.

The thing is, the head gasket on my Subaru was replaced about 30k miles ago. (Now the car is at 144k). So, a relatively new HG.

Any help in diagnosing this would be great. Do I need to just do a coolant flush and burp?

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    If the coolant spewed out in the engine compartment, that's a good thing -- it means it wasn't coming out into your engine oil or exhaust, i.e. the problem probably isn't a bad head gasket again. But driving it hot could ruin the HG again. You'll want to check out all the usual possible causes for overheating. I don't have time to write them up now but there should be existing good answers on the topic.
    – R.. GitHub STOP HELPING ICE
    Dec 11, 2016 at 18:33

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What you describe sounds the cooling system failing to hold pressure. There are two obvious causes of this:

  • A failed pressure cap or its gasket. This would cause low pressure in the cooling system and allow the coolant to boil.

  • A leak between the combustion chamber(s) and the cooling system, the most likely place for this to happen is at the head gasket. Since the pressure in the cylinder is much higher than in the cooling system most of the leak will be combustion gasses into the cooling system.

Sounds like your next step would be to go down to a good auto parts store with you pressure cap and have it inspected/tested. If the cap passes, ask about a test kit for detecting exhaust gasses in the coolant.

While the head gasket is relatively new, it could be failing prematurely due to a poor job of replacing it (and/or of figuring out the cause of the first failure).

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  • Thanks much! Though it might not be the answer I want to hear.
    – tjgrist
    Dec 11, 2016 at 18:52
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    I'd focus on the pressure cap, they're not supposed to let go like that. Either it wasn't on properly, or it failed. Either way, the cooling system isn't getting pressurized, so your coolant gets to boil at a much lower temperature and leave the system. This is what caused your high temps.
    – tlhIngan
    Dec 11, 2016 at 18:59
  • As @tlhIngan says, focus first on the cap both because it is cheap (especially if it was just on wrong) and easy to test. You definitely want to eliminate that possibility before digging into anything more complex and expensive.
    – dlu
    Dec 11, 2016 at 19:09
  • How long ago did you get the head gasket replaced? Could it still be under warranty?
    – dlu
    Dec 11, 2016 at 19:10
  • Head gasket was replaced maybe 4 years ago, so probably not under warranty. Will check. @tlhlngan I feel I should clarify that it was the coolant overflow container cap (on the semi-opaque white reservoir) that popped open and must have been spilling coolant all over the engine. So, the pressure cap that you guys are talking about is the cap on the radiator, which was still on when I stopped.
    – tjgrist
    Dec 11, 2016 at 19:20
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As it turned out, the car needed a new thermostat and a new pressure cap. So, these respondents were on the right track for my particular problem.

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