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My 2006 Subaru Outback overheated when my wife was driving. I got to the car that evening and its radiator was pretty low. I refilled it, and the car ran fine for a week. Then it overheated again. Our mechanic says we have a blown head gasket. We need to wait before we can pay for that repair, so we took it home for now.

I drove it today. It ran fine for a few hours, then rapidly started getting hot. I opened the cap and let it boil, filled the radiator (it took about a half gallon), and kept driving. It overheated again in about five minutes.

This happened about four times over 15 miles - It would suddenly get hot after approximately five minutes of easy driving; I would open the radiator and let it boil off; refilled it with water; and kept driving towards home. It had just started getting hot again when I got home and parked.

Anyway, here's what is confusing me. After it cooled off, I opened the cap (without it needing to boil off), and the radiator was still full! Perhaps most of the coolant I lost on the way home was simply due to my opening the hot radiator and letting it boil... It does lose a bit of coolant even when it isn't overheating, but that might be a hose leak.

Can this behavior be explained by a bad head gasket? The system can obviously hold pressure. I'm wondering if the thermostat is bad instead. Maybe both problems exist.

Thanks for any insight.

  • definitely has symptoms of a bad head gasket. get under the car and look up at the engine. subarus often leak coolant/oil at the bottom corners of the heads when the gaskets go bad. – user4546 May 15 '15 at 23:36
  • Don't know about Subaru, but some engines need to have a procedure done to bleed air out of the cooling system after coolant was emptied and refilled. Otherwise cooling of the engine may fail even with enough coolant in the system. You'd also want to make sure that the coolant pump and its drive belt are fully working before replacing the gasket. – JimmyB Mar 10 '16 at 11:29
  • My fiat punto did this .I think that the high pressure exhaust gasses get into the cooling system but little or no water gets out .In other words the exhaust pressure greatly exceeds the cooling system pressure.When the cooling system is full of hot gas which has a much lower heat capacity than water the moving bubbles make the temp guage move wildly. – Autistic Mar 24 '16 at 6:52
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Yes, it's possible. Also, the bit you lost that you think may be a leaking pipe is actually being turned to steam and blown out your exhaust. The car runs fine until the engine gets hot enough for the thermostat to open up, at which point exhaust gases are let into the radiator. The details of how/why is a bit long, but this is what happens. How do I know this? the 2006 Outback uses the same engine as the 2006 Forester and Impreza models. And those engines all had headgasket problems. My Forester did exactly the same. The symptoms are always the same:

  • Overheating
  • sytematic loss of coolant
  • pressure in the radiator
  • bubbles in the radiator when the thermostat opens

The headgasket is feeble and tears between the cylinder and waterjacket, sucking coolant into the cylinder and blowing some exhaust into the coolant system. An updated headgasket should be installed.

If you want to make dead sure that you don't have any cooling problems again, you should save up some money and have a mishimoto radiator installed in place of the floppy plastic one you have currently. It's not urgent, but it should be done before 120 thousand miles/200 thousand kilometers.

  • Thanks! I guess I'd better just suck up and get it fixed :) – bitsmack May 16 '15 at 2:08
  • can you please expand on how a better radiator will alleviate a head gasket problem ? – amphibient Sep 18 '15 at 16:55
  • The OEM Subaru radiator is a known defective unit that will burst sooner or later. That means no coolant and thus extreme temperatures. Extreme temperatures lead to warped heads which will in turn lead to a blown gasket. – Captain Kenpachi Sep 19 '15 at 18:05
  • It's also too small to adequately cool your car when you're stuck in traffic on a hot summer day with the aircon on. I know this from experience. The Mishimoto radiator is also about twice as thick, meaning there's more space for coolant. – Captain Kenpachi Oct 12 '16 at 10:13

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