Over the past couple of months, I have noticed that the coolant temperature on the highway doesn't reach 82C anymore like it did in the past. It stays around 79C to 80C max.
On high load like driving uphill or in stop and go traffic, the temps still go above 82C until the fan kicks in as it always has been in the past.
But on low load or driving on the highway, it doesn't go over 80C anymore.

I had prophylactically replaced the thermostat in 2019 (about 80'000 km ago) with an AC Delco 131-158. Could it be the thermostat? Or something else?

 

Try burping the coolant system, perhaps replace the coolant if you haven’t since 2019

 

82C is not too low 80C is. when idling does it go above 82C?
I suspect he t-stat.

 

Thank you for your replies.
I replaced the coolant last year. Yes, may try to burp it again.
When idling, it does go above 82C, up to 87C -> I didn't wait any longer.

 

The fan should turn on at about 104C.
If you can get a laser thermometer you could test the temp sensor by pointing at the t-stat housing.

 

So the thermostat it was. I put in a new AC Delco one. And now the temperature on high-way speeds sits again at 82C/180F where it's supposed to be.
The original thermostat was good for at least 12 years. It was still working fine when I replaced it pre-emptively when I changed the coolant back then. The replacement part barely lasted 6 years...

On a sidenote: I am wondering where the temperature needle is pointing at on the "original" American dashboard for 82C/180F? On my dashboard, the needle at that temperature level sits a tiny bit below the third marker. At 86C/187F, it points exactly at the third marker (the angle in the picture is a bit deceiving).
I can't remember ever having seen the needle going up to the fourth, "fat" marker. But it could be that I just didn't pay attention.

 

The gauge needle doesn’t move very much , I use the DIC readout.

 

Think of the analog gauge as an "idiot light"; when it gets to the red it is too late.

 

Thermostat is probably starting to open too soon. Not a big deal yet. It gets any worse, you'll want to change the thermostat.

 

perhaps read post #6 , he did replace the thermostat