Chevrolet Sail: Confused By Cooling System Behaviour

[sacbiker]

We have a Chevrolet Sail Petrol in our home. It doesn't have a radiator cap, instead it has an overflow tank (instead of an expansion tank) with a pressurised cap (20 psi)
I have never seen this sort of set up 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 in to the tank. There are two houses connected to the tank, one coming in and other going out at the bottom.

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

 

[sacbiker]

There is coolant circulation in the overflow tank even during a cold start. Does this mean the thermostat is stuck open?

 

[sacbiker]

*Update*
This is in fact how the system works. The coolant is always in circulation between the radiator and the overflow tank as soon as the engine is started. The thermostat only controls the coolant circulation into the engine block passages when it goes higher than 93 deg C.
I used an OBD scanner to monitor the coolant temperature of the car since it does not have an inbuilt temperature gauge.
If the thermostat was not working, it would have taken a long time before the engine warmed up.

 

[SpeedForce]

What coolant does you use? Is it okay to use a green one?

 

[Redex]

Hi,
You have a perfectly common set up. The system has been used since late last century. I am surprised you have never seen it before. These days most systems are topped up via the expansion tank, even completely filling the system after replacing the radiator, water pump or thermostat etc. Even if you have a radiator cap you can still fill through the expansion tank.
The cooling system is not circulating through the expansion tank as such. There is often a small pipe that vents into the expansion tank that allows any air to bleed into the tank. This also allows you to confirm that the pump is working, because you can see the flow into the tank.
Due to the engine/radiator placement being as low in the body as possible, it is often difficult to reach the radiator cap ( even if it has one fitted ). This low radiator position also makes it difficult to bleed the system of air. You will see that most expansion tanks are fairly small and fitted in a position higher than the engine. This greatly improves the ability for the system to self bleed.
Coolant will flow through the bleed pipe into the expansion tank as soon as you start the engine, thermostat open or closed.
Refer to the user/service manual and only use the grade of coolant specified.
I am old school (67), I dislike the trend to do away with volt meters, oil pressure gauges and temperature gauges. Every vehicle I have has a temperature gauge. When not fitted as standard I add one. I like to see any problem starting as early as possible and not wait for a light or message to tell me the engine is over heating, I also want to know how hot !!!
Even my motorcycle, a single cylinder air cooled Yamaha 600cc XT has a temperature gauge which I fitted. It actually gives me the oil temperature in the dry sump header tank.
I understand that charging, temperature and oil pressure problems are so rare that the gauges have been done away with for cost saving. I would prefer to see the "vital signs" of my engine all of the time !!!! That way I can see a problem developing and deal with it before a breakdown or long term damage.

Regards Neil