Coolant on the carpet, from where?

[misha]

...Hi,

If it is not sealed then how does it build up pressure?

What pressure...there is virtually no pressure there...it's something very minimal.Sealed system is ac system,for example,but coolant evaporates to some extent.
When pressure is higher than it should be...it escapes via expansion cap.
If it's higher than expansion cap can handle....it starts blowing hoses,radiator,headgasket.

[MrAl]

...Hi,

If it is not sealed then how does it build up pressure?

What pressure...there is virtually no pressure there...it's something very minimal.Sealed system is ac system,for example,but coolant evaporates to some extent.
When pressure is higher than it should be...it escapes via expansion cap.
If it's higher than expansion cap can handle....it starts blowing hoses,radiator,headgasket.

Hello again,

It must build up pressure because that is part of the way the boiling point is kept down.
It's something like 14 psi isnt it?
That does not seem like much, but if it was an open system it would spray out like Niagara Falls
Mine does not have an expansion cap.

[Cees Klumper]

I've always understood that, if you open the expansion tank or the radiator when the engine is still hot, like right after shutting it off, the coolant will start to boil and come out. So i've always assumed the system is pressurized. But I can't reconcile that with that overflow hose that is on most systems.

[misha]

...Mine does not have an expansion cap.

What do you think you have on top of expansion tank?!Expansion tank cap.Grey or green one.

Let me say it like this...when you change your coolant and when you start the car to bleed the air from system untill cooling fans kicks in....coolant doesn't escape from expansion tank...except you overfill it.

Of course that there is some pressure in the system but that is something minimal.

[misha]

I've always understood that, if you open the expansion tank or the radiator when the engine is still hot, like right after shutting it off, the coolant will start to boil and come out. So i've always assumed the system is pressurized. But I can't reconcile that with that overflow hose that is on most systems.

You're right.When engine is hot,you always open the radiator cap cap or expansion cap while engine still running.
But even if you open it right after engine is turned off you will have a slightly spillage of coolant for about of second or two.If you open it cafefully and releasing remaining pressure slowly while you open it,that minimal pressure will escape.
This works for healthy cooling system with normal operating temperatures.That said....you don't want to remove radiator or expansion cap when engine is overheating or allready overheated.That requires different procedure.

[abscate]

The system will be at about 1 atmosphere pressure at full operating temperature. Volvo test calls for no leaks at about 1.5 atmosphere pressure

If you open the cap after a hot run, it will spew unless it's leaking badly

[MrAl]

...Mine does not have an expansion cap.

What do you think you have on top of expansion tank?!Expansion tank cap.Grey or green one.

Let me say it like this...when you change your coolant and when you start the car to bleed the air from system untill cooling fans kicks in....coolant doesn't escape from expansion tank...except you overfill it.

Of course that there is some pressure in the system but that is something minimal.

Hi again,

That does not look like an expansion cap, but then i have not examined it in detail which i will do next time.
But if it is an expansion cap, where does the liquid or steam come out if the pressure gets too high? I dont see any holes in it but i will look closer.

Saying that the pressure is minimal does not tell us anything though as we need to know what it really is, 1 psi, 2 psi, 3psi, etc., and i think it is around 14.7 psi or a little more like Steve suggested. That means it will spew out if the cap is opened. If you have ever opened a more standard radiator cap you know it will spew out, so that means there is pressure, and that means that fluid would come out if it was not sealed. If it is an expansion cap then it does not normally leak anyway right? That's unless it is defective, which i guess is a possibility.

So i guess what you are saying is that if the pressure goes too high it will somehow come out the 'expansion' cap. That's fine, but the cap on mine does not look like an expansion cap so where exactly does the fluid come out of that cap if the pressure gets too high?
But even up to that point the system would be considered sealed, albeit conditionally, otherwise coolant would escape all the time when hot.