On Mercedes, the thermostat is also responsible for closing the radiator bypass and directing all the coolant through the radiator. Reinstall the correct thermostat and see if there is a difference.

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Ali Al-Chalabi

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