[Targalid]

I have seen this identical experiment done with cooking oil. Try that and see what happens at the temperatures you are seeing on the gauge, namely above boiling water up to 240, about 115C. (Wear an insulated glove.) Does it open all the way then? It looks partially open. Is there a piece of debris that prevents complete opening? Is it possible to use a pair of needle nose pliers to push it past that point while immersed to see if there is a rough spot on the valve? I would certainly be thinking seriously about replacing this part, about $40 if I recall. Amazing to think that the entire cooling system depends on this little part opening completely. I am really curious about this because I think I have the same thing and will have to pull the engine to do the same experiment.