Overheating Subaru Outback
 

So my wife's 2001 Subaru Outback 2.5 H4 has been overheating lately. The onset of the problem was on a recent trip. I got a P0420 (I think this is unrelated) and the car started overheating about two hours later. I since cleared the code and it hasn't come back. After the trip I discovered that the overflow tank had a lot of fluid over the full line. I replaced the thermostat, radiator, and radiator cap but the problem persists. The coolant was milky, which is indicative of a head gasket problem, although the current head gasket is only about 20k miles old. Any suggestions on next steps with this car? Will fixing the head gasket stop the overheating, or is the head gasket leaky because of getting too hot?

Confirm that it's the head gasket (there's a couple of different methods) and replace it if it is...
And check that your water pump is flowing properly...I've heard of plastic impellers disintegrating. If it isn't pumping worth a dang, it's just going to overheat again and kill your head gasket.
Running lean can cause overheating as well, which might mean it's related to your P0420 code, and all your other efforts might not fix the underlying problem.

Bummer if it's the head gasket at those few miles, but it sounds like water is mixing with oil. Subie head gaskets normally last 100k, right?

I don't know why Subaru and owners are ok with this fundamental flaw that has existed since the 90s.

If you go in to replace the head gasket again be sure to inspect the head carefully. Sometimes aluminum heads will corroded causing a pathway which a new gasket may fix temporarily.

Part of the problem may be that I installed the thermostat on backwards. The thermostat is normally mounted upside down, which I think is a pretty bad design as it allows for air bubbles to get trapped there. I'll see if this ends up fixing the problem.

I think the head gasket should last longer than 100k miles in most cases. Very annoying that this is a common problem in subarus. I no longer think it is the problem though, but we will see.

I think this overheating may have helped me get better mileage though 😂
31 mpg was my last tank.

The overheating problem has been solved! All it took was a correctly installed thermostat with a jiggle pin, with a very good burping of the system.

...and hopefully I do not have to think about doing a Subaru headgasket for about another 100k miles.