Something could have kept it open like a piece of old gasket material or RTV. Like anything else, thermostat'* do fail.

Hey don't get me wrong, I'll be thrilled if the T-stat is the only issue! I just found it strange that the one I removed appears to operate correctly.

I didn't really specify here, but I'm wanting to rule out (or in, if necessary) oil in the coolant and/or coolant in the oil. When I bought this car a couple weeks ago, the coolant level was a little low and I topped off... so it'* going SOMEWHERE but I haven't experienced any rapid coolant loss yet. I'll be keeping an eye on it for any possible LIM gasket issues.

the reason it gets hotter with the thermostat removed is because the coolant is not in the radiator long enough to cool it. some people remove theirs when the engine is overheating and it doesnt help, sometimes it makes it worse

edit; nevermind, realized thats not what you are talking about

^^ Yeah sorry, if that wasn't clear. The thermostat I tested HAD been removed from the car, but it was replaced with a new one. I did not run/test the engine with no thermostat installed. (I know you realized that jw, just clarifying for anyone else reading.)

Just a quick update, that over a month later my Buick'* heat is still working fine. Luckily for me, the thermostat fixed this problem in particular and it wasn't just a 2-3 day solution (as mentioned on page 1).

I still have a small coolant leak somewhere (I can smell burning coolant when engine is hot, and top it off a few inches in the reservoir tank about once/week), but that appears to be a separate issue.

Small leak like that, have you checked the throttle body gasket? Crossover pipe runs underneath it and could be the cause of the smell.......they tend to leak more on a cold engine....then as things warm up and expand, they seal.......run your finger under the joint between the throttle body and the upper plenum, just a few minutes after the car has started......

Small seep on the LIM gasket could be evaporating too. Looky lou into the darkness with a flash light . The ends of the heads usually will be wet.