You should rule out turbo first. Perform a coolant system pressure check.

 

Good idea. I'm not familiar with the configuration of the turbo. Is there a way that you could introduce oil to the cooling system via the turbo?

 

Yes. The CHRA is both oil lubricated and coolant-fed. At 180k miles, I'd be surprised if turbo seals weren't leaky.

 

Would you consider this to be a more common failure than a head gasket? Can't say I've ever heard of a turbo seal failure mixing oil with coolant.

 

It's more common to see additional signs like oil burning and consumption, but it can happen. I would rule it out first. Seal failures were common on K04 turbochargers and this is exactly the failure that occurred on my wife's old MS3.

If I were in your shoes, I would perform a coolant system pressure test first, then inspect the turbocharger for signs of failure (you'll typically see oil residue and corrosion on the turbine and sometimes the compressor). If the system pressure test fails AND the turbocharger looks to be in good condition, suspect head gasket.

This part is important: the internet loves to poke at Subarus for head gasket failures, but this wasn't actually a common failure in any WRX or STI. It was a problem with the single-layer gaskets on the SOHC 2.5L NA motor. The later-model Impreza and all WRX/STI are just as susceptible to HG failure as any other brand at stock output.

 

4. Any advice on good sources for good quality gaskets and other parts? Is OEM the only way to go, or is something like Mahle a good option?

I'd only select OEM on the critical gaskets/parts not easily accessible later on (e.g., rear main, head gasket, etc.). Aisin is good for your timing belt, water pump, etc. items there if you skip OEM.
Mahle is fine in my experience. I use to like Fel-Pro but have not had great luck with their stuff post-COVID. Dorman seems to always be a 50:50 shot on whether it lasts, with the exception of their "OEM improved" parts I really like (such as their metal-instead-of-plastic connection GM heater hoses).

As for "sources", I buy all my OEM parts at the below site. *BUT change the dealer you're buying from when doing local pick-up to compare prices - they are all different by dealer. Also, they do not honor the quoted online price in-store if you just walk-in; upon learning this recently, I literally stood in front of the guy at the counter and ordered on my phone, saving $30 on a pair of fucking half moons LOL.

Subaru Parts Online from Subaru of America

Buy Subaru Parts and Subaru Accessories online from your local Subaru retailer. Love every mile with OEM Subaru Parts and Accessories.

parts.subaru.com

 

I'll go ahead with the system pressure test. Seems that would be a good first step.

However, I'm having a hard time getting on board with the turbo leak theory. In all the turbos I've dealt with, the coolant and oil passages are separated by the CHRA casting; the seal condition wouldn't have any bearing on whether or not the two fluids mix. In that situation, the only way to get oil in your coolant via the turbo is if the housing was cracked. Am I correct, or is there something unique about the turbo on the 15+ WRX that would allow this to happen?

Don't get me wrong, I think I'd prefer the issue to be the turbo instead of a head gasket, but I'm having a hard time working through exactly how that would happen.

Thanks,

Marshal

 

However, I'm having a hard time getting on board with the turbo leak theory. In all the turbos I've dealt with, the coolant and oil passages are separated by the CHRA casting; the seal condition wouldn't have any bearing on whether or not the two fluids mix. In that situation, the only way to get oil in your coolant via the turbo is if the housing was cracked. Am I correct, or is there something unique about the turbo on the 15+ WRX that would allow this to happen?

You are correct to question.

This depends on the bearing and cartridge design. I'm not sure about the design of the Garret turbo on the FA20F, but best to rule it out before tearing the motor open.

 

#2 has nothing to do with EJ or subaru. Any relatively modern engine head that has been on for a few years needs to be machined flat.