So, now I'm wishing I never pulled the head off this car....
Yep, new head gasket's got problems. Leaking coolant externally. And the timing couldn't be worse, since I'm almost completely moved out of my house with most of my tools in storage for at least a month and a half. Not much asphalt in farm country, so going 'around the block' is 18 miles, and I lose about half a cup of coolant. But only when hot, seals right up again once it cools down.
Basically, I can summarize the likely failure cause pretty easily. The original factory gasket was a graphite composite, meanwhile nearly all the replacements out there (including those from Toyota directly) are MLS steel. I'm guessing that since neither sealing surface was prepped to the standard needed for an MLS gasket, it's just not a compatible combination. I did the best I could to make sure everything was clean and flat, even checked the head for straightness on a calibrated surface plate. Didn't use any power tools on anything, all by hand, and there were no pits, gouges, dimples or anything that I could see or feel on either surface. It simply needs a graphite composite gasket unless I do a full teardown and have both the block and head resurfaced to the tighter standard. And before you say I should have had the head resurfaced.... it's leaking out of the BLOCK side of the gasket, not the head side. The gasket extends out a bit in that area, and the topside is dry where the bottom is wet.
I've got a couple plans of attack.
Plan A, and this one is the most controversial, is a 'miracle in a bottle'. Normally I'd never consider it. However, given the color of the coolant that I removed when I flushed the system, there's a good chance it's already had a dose at some point. The stuff was a deep cocoa brown color, well beyond the normal red or orange of a coolant. That and next summer I plan on starting the rebuild of a new engine anyway. I'm going to give Subaru Coolant Conditioner a try, which is a product called Holts RadWeld with a Subaru label on the bottle (which still has the word Holts molded into the plastic). Main reason I'm choosing this one is it's the safest choice I can find. Believe it or not.... it's mandatory for dealer service by Subaru, nearly every Subaru that's had fresh coolant at a dealership in the last 10 years has it in the system. You actually void your warranty if you DON'T use it. I'm also seeing clean oil, and clean coolant, so the two aren't mixing at all. That plus no symptoms of coolant in the cylinders either. Plugs looks normal and no white smoke. Very low risk but I also know there's a good chance it won't work.
Assuming it doesn't work she's getting a fresh graphite head gasket at the first opportunity. Plus I'll have the whole cooling system flushed at a shop where they can really clean it out. If it wasn't for the fact I just have no ability to work on it right now I'd go with this plan right away. Just trying to work out how the hell I can get this done in time for me to get on the track this summer. If I can't get my track time the whole timetable gets pushed a year.