My DIY friends sometimes ask me if they should install a “summer” thermostat in their vehicle (or remove the thermostat altogether) to help it run cooler in hot weather. My answer is always the same—absolutely not! When you start up a cold engine, the computer immediately checks the engine coolant and outside air temperature sensors and ignores the readings from the oxygen sensors. This is called “open loop,” and it’s the least efficient in terms of gas mileage and emissions. That’s why carmakers want the engine to heat up to full operating temperature as quickly as possible. So most install a 195-degree thermostat at the factory and program the computer to switch over to the more fuel-efficient “closed loop” mode when it reaches that temperature. If you remove the thermostat or install a lower “summer” thermostat (180 degrees), the computer will never go into “closed loop.” Your vehicle won’t run well and will suffer poor gas mileage. The richer than normal fuel mixtures can also cause damage to the very expensive catalytic converter under your vehicle.
So if your vehicle experiences overheating problems in the summer, fix the root cause (water pump, clogged radiator, inoperative cooling fans, bum fan relay, etc.) instead of masking the symptoms with a lower temperature thermostat. If you’ve already pulled out the thermostat, turn it over and check the temperature stamped into the bottom of the copper pellet. Then check with the parts store to see what temperature thermostat your engine requires.
— Rick Muscoplat, Automotive Editor
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