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The very first thing I would do is replace the thermostat. This is a very inexpensive part and can very likely be the problem. Even though the symptoms are the same, it doesn't mean the same problem is occurring. For the record though, I do totally understand your concern.
If the thermostat doesn't resolve the issue, then you may be looking at a head gasket problem. New engine or not, it can be happening. If you have a warranty hopefully it will cover it. A combustion test will need to be done to see if there is any exhaust gas in the coolant.
It does sound like a bad thermostat, so I would start there.
Also, make sure the cooling fans are both going when the engine is hot.
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On these cars, when the head gasket fails, it allows the exhaust into the cooling system. This overheats the engine so fast that the thermostat can't open fast enough to get the hot coolant into the radiator. This is why one hose is hot and the other is cold. The same as the thermostat going bad. If you replace it, and it doesn't solve the problem, it's likely the head gasket. A combustion test will prove that. They should have done that when they had your car before.
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No, as I explained, it heats up from the 1500 degree exhaust gas so fast the thermostat doesn't open before the car overheats.
I personally think you have a bad thermostat to be completely honest here. This type of head gasket issue on this particular engine isn't very common. Usually they leak externally at the rear driver side head.
I am hoping it's the thermostat as well. I know the head gaskets aren't cheap to do, so hoping you have some warranty on this engine if it is in fact the problem. The thermostat will stop the coolant from transferring into the radiator as well, so it may feel cool. Remember, it's the coolant in the engine that's overheating, not the radiator.
Do the combustion test, you have a bad head gasket sorry to say.
Again, the hot exhaust is heating the coolant before the thermostat can open. Take the thermostat out of the car, then it will flow freely and less chance of it overheating because at least then the coolant can make it to the radiator. I know you don't want to hear the head gasket is the problem. I totally get that, but throwing parts you don''t need is only going to cost you money that you could put towards repairing the head gasket. I have been down this road with these cars so many times, I have seen it time and time again.
No, I have left them idle for an hour in the bay and never showed a problem, then take it for a drive for 3 miles and its overheating. I have also had some that people got mad because they said it was overheating and I couldn't get it to!!
Then drive it 25 miles and bam! Overheat.
This is why I suggest the combustion test. It's a chemical test that shows the hydrocarbons if they are in the coolant. Another thing, you clearly know what the coolant smells like since you have been working with it. If it smells acidy and not sweet, it's another indicator. I don't recommend it as the "main" test, but sometimes you can tell by the smell. I assume the cooling fans are kicking on when it's hot?
Being a Subaru there isnt a lot of logic on the head gasket issue.
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