Yes, always use the correct thermostat. Do not install a cooler thermostat in an attempt to make the engine run cooler. All it will do is regulate the bottom operating range of the engine to be cooler, but will obviously have no effect of the upper range of the engine.

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Ali Al-Chalabi

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