EDIT - SOLVED. It turned out to be a warped head. Head gasket looked OK, but was replaced anyway of course. Head was not cracked, just needed to be re-surfaced. I did a ton of other stuff too while I had it all apart.

Here's the story chronologically.

2001 E53 3.0i, 185k miles, owned for 3+ years now, running GREAT.

2 weeks ago my son was driving it home from a school graduation party in the middle of the night. Upper radiator hose sprung a leak (the plastic at the bend where it goes into the radiator) and a lot of coolant drained. He now knows he should have just stopped right there. But he pulled over, walked to Safeway and got a gallon of coolant, added it and drove the ~3 miles home from there. He says he did not notice overheating, and I don't know how bad it was before or after this incident. I think there's a good chance it overheated and he did not notice it.

Once home, it took 6 quarts of water to fill. I replaced upper and lower radiator hoses and a new radiator for good measure, all OEM (Behr, Rein, BMW).

It tested OK, I thought, then my daughter needed it for a 400 mile round trip to Lake Tahoe. She had issues starting about half way there with low coolant. She added coolant as needed, and made it home without too much issue. I think she was burning through about one quart of coolant per hundred miles.

Testing all day today ...
Engine cold, I can pressurize the system to 23 psi with no leaks at all. Bone dry, holds pressure for a really long time. I did not initially go that high, but after the other tests, I went that high to check if the reservoir cap might be opening too early (nope).

After a quick 0.5 mile test drive around the 'hood, with the hood up and engine idling (and warmed up), coolant flowing out of the reservoir cap (at the 2-bar pressure release point). Cap is Rein, a year or two old, works fine as far as I can tell. 2 bar release pressure, tested to 23 psi as noted above.

Lisle 75500 Combustion Leak Detector gave a positive result after about 5-10 minutes. The blue turned to yellowish green.

No oil noticed in coolant. I have not checked the oil for coolant yet.

The SUV still runs PERFECTLY other than this. Revs great, idles smooth, no codes, etc.

EDIT - it does NOT overheat. My daughter, on her 400 mile trip was told to pay very close attention to the gauge. And I can confirm I've never seen it go past 12 o-clock. So the damage seems to have come from that one-time event.

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Does this sound like a head gasket? Worse? Anything else I can do to test things before tearing into it?

Any other ideas for a problem that would give these symptoms (no coolant leaks other than radiator cap release, combustion gases in coolant, coolant overpressures when driven)?

I'm thinking to do a compression test (at least) before tearing into it. If I find problems I may do a leakdown test too. After that, it's probably time to start taking things apart until I can find the actual failure point. I've got a driveway, a garage full of tools, but no lift.

I really don't need another project, but if needed, I would do everything to fix this myself. I've rebuilt simpler engines back in the day, and have plenty of good general purpose tools. No special BMW tools, though.

I spent a couple of hours searching for info, and there is not much on this that I found. I guess I'd follow the Bentley manual.

If it ends up being a bunch of "work" taking stuff apart, cleaning it, replacing things, sending the head out for machining, etc. I'm very happy to do that sort of stuff. But if it ends up being a logistical optimization to figure out what to do, what special tools to buy vs. hack something and take a risk, etc., maybe spending $2,000+ and wondering if I should just punt. That's not so much fun for me.

Any advice would be appreciated.

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2001 X5 3.0i, 203k miles, AT, owned since 2014