Do not install a lower temp thermostat. It will do nothing to control the top end of the temp range, it only controls the min temp. Running cooler than the car is designed is not a good thing. If it really is running hot, your problem lies elsewhere.

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Ali Al-Chalabi

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