2) Pure water boils at 212.... I need more antifreeze in the thing.

That's the cause. The cap is also audibly venting a little pressure, so I'll upgrade, but mostly it was a lack of antifreeze.

It happened like this:

1) changed thermostat and didn't allow car to fully warm up before driving. Created an air bubble.
2) Replaced missing coolant with distilled water, which ruined the anti-boil properties.
3) Coolant repeatedly boiled and was refilled with distilled water until it was pure water.

Fixed this by letting some boil out (lazy way) and refilling with straight antifreeze.
Tested by letting it idle for 30 minutes and with some engine revving and monitored coolant expansion after shut-off.


Happy Bricking, hope this ends up helping someone.
Will
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1990 740 Turbo, on its way to stock specs, maybe beyond