Vue 3.0 thermostat

Temp gauge is way low. Old thermo looks exactly like new one. Should there be an obvious flaw?did u know Opel puts the thermo under the lower intake? 4+hr job

Not generally.
But there is a simple way to test a thermostat. The temperature at which the T-stat should open will be stamped into the T-stat. Get a hotplate, a tempered vessel that will withstand the heat, some water, and a thermometer or temperature measure device (like a thermocouple) that’ll measure the appropriate temp. Using a piece of wire, hang the T-stat in the water with the thermometer (or whatever). Heat the water to the temperature indicated and see if the T-stat opens. If the T-stat opens way too early, the engine will run cool.

Good Grief , even I know that you don’t judge a thermostat by appearance.

sitting there on the bench (and the angle we are looking at it,) it sure looks like one is closed more than the other…

You don’t show the two thermostats side by side with the same lighting and orientation, so hard to say. The ones I’ve had that failed slightly open were noticeably distorted in appearance and clearly not sealing 100%. But there are other failure modes I haven’t seen. It could look ok at room temp for example, but start to open at 125 degrees. That would produce your symptom too. TSM’s test above is what I always do with the new one anytime I’m replacing a t-stat. I test the old one just for kicks at the same time. I’ve found bad replacement t-stats using that test, right out of the box. measure both the opening temperature and the dimension it moves.

the thermo plunger is seated against the bronze colored “disc” in pics. Does the thermo plunger retract during operation?

When heated, the plunger comes out of the removable part of the t-stat housing thus opening the coolant path to the radiator and closing the recirculating path used during engine warm-up. So, it should be seated against the “disc” when the engine is hot.

The closed valve in the thermostat is inside the housing, we can’t see it in the pictures. When the thermostat opens the disc on back of the thermostat blocks the by-pass port in the engine.