I see so many cars, specifically older ones (< 2005), which have their thermostats removed.

Mechanics seem to love throwing out the thermostat to fix an overheating engine. It’s mostly because we believe the only purpose of it is to let the engine warm up quickly and prevent it from going cold during driving.


There are different types of thermostats but that isn’t what we’re going to talk about.

It has the following two purposes:

  1. On cold start, help the engine get to operating temperature quickly
  2. Keep the engine temperature in the optimal operating range.

It achieves the above with just one simple mechanism:

it blocks coolant from going into the radiator until the coolant’s temperature reaches a specified threshold.

Cold start

On a cold start, the coolant is going to be lower than operating temperature, so the thermostat will remain closed and prevent coolant in the engine block from going out into the radiator to be cooled. This will keep the coolant in the engine block and raise it’s temperature as the engine block starts to heat up with the combustion cycles.

Once the engine block, and coolant along with it, get up to operating temperature, the thermostat will open up to let the coolant now flow through the radiator to cool down.

Keeping coolant at optimal temperature

Once the coolant is at operating temperature, the thermostat opens up to let the radiator cool down the coolant.

As it cools down, the thermostat starts closing back up. This prevents the coolant’s temperature from dropping below optimal temperature.

Since the engine is running, it will raise up the coolant temperature again, and the cycle will continue.

Coolant’s flow as state diagram.

	state "Engine block" as eb
	state "Thermostat" as t
	state if_state <<choice>>
	state "Radiator" as r

	eb --> t
	t --> if_state
	if_state --> CLOSED: coolant temp. < min. operating temp.
	if_state --> OPEN: coolant temp. >= min. operating temp.
	CLOSED --> eb
	OPEN --> r
	r --> eb

The other important function

Most of the people already know this. It helps warm up the engine and keep the engine in the operating zone.

But I want to share my experience with a car that I drove that had it’s thermostat removed (bear with me, I like stories). We’ll then see that the thermostat has another, very important function.

I was travelling to murree, and got stuck at an incline full of traffic. It was a manual car so I had to stay on the clutch for a little longer as traffic moved bit by bit.

Murree story uphill

Suddenly, H appeared. The car was starting to overheat. Nani? I resorted to my well-trained reflex of turning up the fans full and temperature to full heat. The sign went way quickly.

Murree story downhill

In another instance, I was going downhill from murree and the weather was real chilly. Guess what appeared now? C. The car’s engine was also chilling, which it shouldn’t. I’m here going 3k rpm on a cold engine.

I stopped and let the car idle. The sign went away.


Engine being cool in cool weather

First let’s discuss why the car was chilling in chill weather. Pretty simple. No thermostat means the coolant is always flowing through the radiator. I was going downhill, the fan’s were not direct (i.e. not always running). But the air itself was so cold that it was alone able to cool the coolant that was flowing through the radiator.

If there was a thermostat, it would’ve prevented coolant from going into the radiator in the first place. Then it didn’t matter if we had the radiator in the freezer. The engine would still warm up.


Now this is the real deal. Remember I mentioned that without a thermostat, the coolant is always flowing through the radiator? Well there’s one more detail which is:

Without a thermostat, the coolant is always free-flowing through the radiator.

Free-flowing means that the entire coolant is just rapidly circulating in the whole system. Now that’s not good.

When it’s free-flowing, it goes through the radiator very quickly. This makes it really hard for the radiator to cool it down. The coolant needs to go through the radiator at a steady place to allow the incoming air or fan to cool down the coolant as it passes through the tubes.

The thermostat does just that. When the thermostat opens up, it doesn’t just let the entire coolant go ham. It keeps the coolant flow regulated.

The thermostat is not a binary switch. It’s modular. As the coolant reaches min. operating temperature, the thermostat opens up slightly. As the temperature increases, the thermostat starts opening up more.

This is the one very important detail that we ignore when removing the thermostat.


If you remove the thermostat of your car, then:

  • In cold weather, your engine will be running cold, which is not good for your engine
  • In hot weather, your engine will overheat even if the coolant is flowing through the radiator

If you have a thermostat and the car is overheating. Check the thermostat. If it is working, then the problem lies in the cooling system (e.g. radiator). Address the cause directly.