Got the new thermostat, but its got the same damn problem as the old one. It rattles in the housing and probably wont seal up to water, acting as if it was stuck open.
The new thermostat indicates an opening temperature of 82 Celsius, or 180 failheit. The previous one appeared to open(reverse tested by putting in hot water and seeing when it closed) at 172F, so if I get this working it should be even better for gas mileage than the OEM thermostat.