Try this: put a thermometer under your hood overnight. in morning, before sun hits the hood do the engine off scan and check all your temp sensors against the thermometer. You'll at least have IAT and ECT to work with.
It used to be possible to install a Tstat wrong. Dont know if it is, now And if it was backwards, the engine heat would overshoot at the least before it opened.
Last ditch is to put it in a pot on stove, with water and a thermometer and note when it starts to open.
I was trying to settle an argument about something else and strapped a digital therm probe on the hose right at the thermostat gooseneck. then wrapped a couple layers tape around it. Never got past 170.
Taking a long hiatus; totally unrelated blather at....
99 Bravadiva - She's SUCH a Good Girl.