You should never, ever remove the thermostat.
I have had several opposing views expressed to me about whether it makes a car run cooler in the summer to remove the thermostat. Does it?
TOM: Yes, Lee, removing the thermostat does make the car run cooler. But you should never, ever do it.
RAY: Everybody knows that it's not good for a car to run too hot. But you may not know that it's also bad for a car to run too cool.
TOM: The thermostat is just a valve that goes between the engine and the radiator. It's purpose is to keep the engine at the proper operating temperature. When the engine gets too hot, the thermostat opens up and lets the engine coolant flow through the radiator, where it cools off. When the engine is too cool, the thermostat closes and stops the coolant from flowing through the radiator, so the engine heats back up.
RAY: In the old days, shade-tree mechanics used to remove thermostats as a remedy of last resort for overheating cars. But that just covered up the problem rather than fixing it. It's sort of like taking aspirin for a broken arm. It makes the pain go away, but it doesn't solve the problem.
TOM: But taking out the thermostat can actually do quite a bit of damage on a modern car. Most cars now have computers, which constantly monitor the engine temperature and adjust things accordingly. If the computer thinks the engine is cold--and it WILL be cold without a thermostat--it'll richen the fuel mixture. That will lead to poor mileage, carbon buildup, and eventually, a $400 repair bill for a new catalytic converter.
RAY: So leave the thermostat where it is, Lee, and find out what's really making your car overheat. Unless of course, you just want it to stay cool long enough for one last trip...to the junk yard.