What is the purpose of a thermostat?

What is the purpose of a thermostat? I’m confused, I’ve heard it’s to regulate temperature but, in the summer you can take it out you shouldn’t overheat being that it’s more for colder climates. I also heard that the thermostat closes once the engine has reached its regulating temperature which allows the coolant in the radiator to get cool. Whether as to remove the thermostat it doesn’t give the coolant in the radiator enough time to cool. Being that the coolant is constantly circulating.

The thermostat holds the coolant in the engine so the engine warms up faster. Then once the coolant reaches a specified temperature, the thermostat opens and allows the coolant to flow to the radiator.


You are not attempting to run your engine as cool as poosible.
You are trying to maintain an operating temperature range.
The thermostat does this, summer or winter.
Proper fuel combustion, lubrication, emissions etc are maximized when within the proper temperature range…not just cooler…but the right temps.

So if i go without a thermostat what will happen?

Your engine will take longer to warm up, the CEL may come on, fuel mileage may nose-dive, and other issues related to that.

Removal of the thermostat was a somewhat viable option in the old days; not so much now.

Say the thermostat is stuck open will that cause a car to overheat if driven for 40 minutes to an hour opposed to if it was stuck closed it would overheat idling before you leave home?

The engine will probably use more fuel.

The engine has to reach a specified temperature before the computer will go into what is called the closed loop mode. This is where the computer adjusts the fuel mixture from the signals it receives from the oxygen sensor.

If the engine doesn’t reach this specified temperature the computer won’t go into this closed loop mode. But instead stay in the open loop mode. So the computer will deliver more gas because the computer thinks the engine is still cold.


In addition to using more fuel, emissions and engine wear will both increase when the thermostat is removed, and the motor oil will rapidly become contaminated with excess moisture and combustion byproducts if the thermostat is removed.

If this was the '50s or the '60s, the OP’s strategy wouldn’t be too far off-base, but it is now a few decades out of date.

Having no thermostat will not cause it to overheat .
If it overheats with no thermostat, you have much bigger problems like radiator flow, water pump, grille/radiator air flow, engine block grease build up, etc.

An engine with a thermostat stuck open should not overheat.

A thermostat can stick closed and the engine will overheat. This can occur at idle, at speed, and may even be erratic in nature.

Is there a reason for the line of questioning; maybe related to a past or current problem?

If the thermostat is stuck closed, the engine will overheat. And this will happen very quickly.

If the thermostat is stuck open, the engine won’t overheat.


I had a Ford LTD with a stuck open thermostat. The engine never warmed up. And the heater was not all that warm. Of course, that car had a huge radiator.

Back in the summer of 1973 I was driving through Arizona and my van was running very hot. So I pulled out the thermostat and continued driving till I found a town with a radiator shop.

They cleaned out my radiator (which was my problem), but they then chastised me for taking out the thermostat. They said in the hot south the thermostat was needed to slow down the flow of coolant to give it enough time to give off heat in the radiator.

I never encountered any situations since then to prove or disprove their claims.

Well my car had start overheating last month. But, it would overheat when i’m sitting still and cool itself when moving. I came home, the next day filled the radiator with water and let it run for a while. I even drove the car around the community I live in still didn’t overheat. Well July 4 it start to overheat this time it would overheat while moving and sitting still. I parked in the parking garage came back 7 hours later filled the radiator up with coolant and proceeded home. Well I believe I forgot to put the cap back on the reservoir but it overheat on the way home. When I pop the hood the cap was off the reservoir boiling hot water and steam was coming out of the reservoir. I pop the cap on the radiator let the hot air or stream out filled the radiator up with water. Drove home, to a friends house, my dads house the next day and it still didn’t overheat. On the way ti work Tuesday which is about a 40 minute drive it starts to overheat right before i got ready to go inside for work. Well when i got off I cranked up and immediately proceeded home it overheat about 10 minutes being hung up in stop lights. I cut off looked up under the hood went back and check the temperature gauge the needle was dropping. So i got back in the car made it about 8 more miles it overheats again. Let the hot stream out filled up with water and it didn’t overheat again. The next day i took the thermostat out and it still overheated. It seems to overheat about 40 minutes after driving. Once you let the hot stream out fill it back up with water it doesn’t overheat the other 40 minutes home. I’ve replace the water pump, timing belt and thermostat later year. And radiator last week.

If it continued to overheat without a thermostat, then that’s not the problem. Since you replaced all of that and ruled out any leaks, you may be looking at a blown head gasket.

“They said in the hot south the thermostat was needed to slow down the flow of coolant to give it enough time to give off heat in the radiator.”

You will always get more heat transfer at a higher flow through the radiator. However, there are two instances where removing the t-stat will hurt engine cooling (and are probably the source of this misconception). 1. The water pump suction on the lower radiator hose is enough to collapse it, thus starving the pump. B. The increased water pump suction lowers the pressure enough to boil the water and “vapor lock” (cavitate) the pump.

You will always get more heat transfer at a higher flow through the radiator.

Is it always? Or is it dependent on too many variables for there to be one answer?

What kind of car? Radiator cooling fan, or fans, operating as they should?

Pretty much always, just like you feel cooler when the fan’s on high vs. low. Heat transfer is more efficient at higher flow rates, per minute (not per gallon flowed).

Until someone can show me an engine that runs cooler with a reduced coolant circulation rate, I’ll stick with always.