Super strange phenomenon - trying to figure out if maybe I'm doing something wrong (or looking in the wrong place).

Maybe someone can explain how the thermostat in these cars work and if they work differently then I expect.

Got a new (to me) 2008 4.8i. When I got the car, it had no heat. After some diagnosis I determined the problem was BOTH in the heater valve and the thermostat. Heater valve seals had failed, and I noticed that the lower radiator hose was very cool to the touch with the engine warm.

Replaced both with OEM parts from BMW (they had both in stock funny enough!) and things worked fine. Heat started working all was well.

A few days later, I noticed a leak coming from the lower radiator hose. Seems like now that the thermostat was working, the old lower radiator hose started to leak. Since the system seemed old, this weekend I did a full cooling system - radiator (nissens) as well as upper and lower hoses as well as expansion tank.

Everything seemed well until today. Noticed heat was only so-so, put in my diagnostic tool and the radiator outlet temp was only 70deg when engine temp was 220. Lower rad hose was cold - AGAIN.

It seems strange (but possible) to have a brand-new OE thermostat go bad so fast. I *assume* these thermostats are heat-controlled and the sensor is just for reading temp? Or is there possible another issue causing my thermostat not to open?

Going to throw in a cheap parts-store unit tonight just to confirm the failure.

Thanks!