You are correct. Specs on a properly operating thermostat start to open at 87C and is fully open at 102C. As they get older, they gradually start to open at lower and lower temps. A new thermostat will bring the engine back up to a bottom line of 85-90C on the highway, where it belongs.

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Ali Al-Chalabi

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