[sac23]

We have a Chevrolet Sail Petrol in our home. It doesn't have a radiator cap, instead, it has an overflow tank

Expansion tank vs Overflow tank

(instead of an expansion tank) with a pressurized cap (20 psi).
I have never seen this sort of setup in other vehicles we have owned.
The coolant can only be added to the system through this cap on the overflow tank.
Everything is normal with the car and coolant levels are also stable.

Recently, for some reason, I decided to rev the engine of the car while the bonnet was open using the throttle body. By doing this, I was able to notice something unusual in the overflow tank of the car. There is a gush of fluid coming into the tank. There are two houses connected to the tank, one coming in and the other going out at the bottom.

Please see the video
It looks chaotic. Is this how it is supposed to work? The coolant is still clear and at the same max level as it was a year ago. The vehicle doesn't overheat. The engine oil which was changed 4000 km ago is still good and not cloudy or dirty.

Is it because the thermostat is stuck open?

[ObsessedByFIAT]

Isn't that how is it supposed to function?

There are two ways the coolant circulates. When the car is cold, the coolant does not enter the block because the thermostat valve is closed. During this time, the coolant only circulates in the radiator.
When the optimum temprature reaches, the thermostat valve opens and that is when the coolant starts to circulate in the engine block.

So in short, the coolant keeps circulating, either in radiator nor tje engine block and this you see the coolant flow in the expansion tank.

In case if you understand hindi, the below video by Mukesh Chandra Gond gives a very good explanation :

https://youtu.be/rfOGSWbLlxY

[fordday]

Is that a brand new car or you maintain the engine bay to this level of cleanliness?

What you saw is normal movement of coolant circulation from the radiator. ObsessedByFiat described it correctly when the coolant flows from engine and when from radiator. Nothing to worry.

The overflow tank cap is pressurized because water will expand and vapourize when heated. So the tank and the cap will be highly pressurized when the engine is in normal operating temperature. You will not be able to screw open the cap. Even if you can, boiling liquid will blow up on your hands and face.

[sac23]

Originally Posted by ObsessedByFIAT

Isn't that how is it supposed to function?

So in short, the coolant keeps circulating, either in radiator nor tje engine block and this you see the coolant flow in the expansion tank.

Thanks for the clarification.

Is that a brand new car or you maintain the engine bay to this level of cleanliness?

No, that photo was taken off google for a better understanding.

The overflow tank cap is pressurized because water will expand and vapourize when heated. So the tank and the cap will be highly pressurized when the engine is in normal operating temperature. You will not be able to screw open the cap. Even if you can, boiling liquid will blow up on your hands and face.

Thank you for the clarification.