There is usually a pretty good spread for where the thermostat starts to open until it is fully open. The temp stamped on the thermostat is when it starts to open. Don't know about the 80C thermostat, but the 87C thermostats are not fully open until they get another 15C warmer. Also, I have noticed that a brand new thermostat sometimes does not crack open the very first time right on temp. Every time I replace an 87C thermostat, I have noticed that it never cracks open the first time until almost 100C. After it opens and closes a couples of times, it starts functioning normally.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record