The typical "low coolant" warning on the big screen is either from:

- Actually low coolant

- The metal gizmo in the coolant overflow tank got stuck. Cleaning the tank sometimes resolves that (or just buying a new tank). One could just short the wires...but that defeats the purpose.

GM sold an overflow tank without the sensor so maybe that feature was not included in Euro markets/early Astras.

There are unbranded overflow tanks but the European forums complain about them leaking after a short period of time. Buy OEM.

 

Is it an overflow tank or expansion tank?

 

I am sure it has an expansion tank with a level sensor in it.

 

Correct - expansion tank with a floating ball "inside" and an external level sensor.

Our 2008 North American Astra had surge tank cap 09202799 (21 PSI). Have dealer confirm yours with your VIN number.

Of course, the european tanks used a variety of cap pressures (1.2 bar, 1.4 bar, etc.) but they typically ran significantly hotter for emissions.

I don't know if newer North American z18xer engines had revised pressure ratings on the Cruze, Sonic, etc.

 

I ask because the Hyundai Kona I rented last year had what appeared to have an unpressurized overflow tank with some pressure valve between the cooling system and this overflow tank. It had a tank with a vertical filler pipe and a "full" range.
The top cap was a plastic press down like the one on a windshield washer fluid tank.

The bizarre part is that the fluid level, when cold, was never the same twice.

I understand that the influx and outflux would likely be different based on rate of cooling, and different residual pressures would be left in the cooling system.
Not sure why the fluid in this tank wasn't boiling when warm....

Why not just use an a simple expansion tank?