The fan clutch shouldn't have any effect on the bottom end reading on the coolant temp.

It sounds as though the thermostat you installed may be defective. You can test it in a pot of boiling water with a thermometer. It should start to open at 87C and be close to fully open at 100C, when the water boils.

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Ali Al-Chalabi

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