The Avant is getting exhaust gas in the coolant. This has been confirmed by shop testing, though aside from discoloration of the coolant there are no other apparent effects.
It made a trip from Seattle to St. Louis without any issue, so I have begun other checks to try to find what is at the root of the problem. Today I was planning to do a compression test, followed by leak down check.
I completed the compression test, recording the following numbers:
Not bad for an engine with 290,000 miles on it.
I was ready to start the leak down check when I realized that the fitting on the pressure regulator was a different size than the hose I used with the compression test, and thus I had no way to connect the regulator to the cylinder head. I now have the correct hose on order so the leak down check will have to wait a few days.