Cooling System Tech. Is the overflow pressurized?

[Pathofinder]

On my 01 Pathfinder, it has an overflow tank that is attached to the radiator with a 3' long hose, and a nipple on the bottom of the overflow tank connecting the the radiator and the overflow.

What Im curious about is, is the overflow ever supposed to be pressurized? And is it part of a "sealed" system??

It has a vent hole near the top of the radiator built into the neck, which appears to be a place for the coolant to exit if the overflow was to get to hot.

So I dont get how it would be a sealed pressurized system with this vent hole there.

Anybody care to explain I would love to know and it may help me understand a problem Im having

[Pathofinder]

[seang]

If I understand this right, coolant expands as it gets warmer, so as the engine reaches temperature, pressure builds and some of the coolant mixture gets pushed past the radiator cap and into the overflow tank. When the engine cools, the coolant contracts, which relieves the pressure and pulls coolant back into the system from the overflow. The overflow tank itself is not pressurized.

[alms24sebring]

I have wondered about this a time or 2 myself, but I think Seang is right. The cap holds up to a certain pressure, I think its 14-16psi. When the coolant gets hot enough, the pressure builds and it pushes the spring on the cap up and the coolant goes into the overflow. When the pressure drops b/c of the extra coolant in the resevior, the cap seals again. If the cap fails or it overheats, the coolant fills the overflow and can pour out the vent at the top. That vent also keeps the res ant equal pressure to the atmosphere meaning not pressurized.

You are still having overheating problems?!?!?! Your waterpump fins might be done then . Or I can simply recommend a new car since thats always easier.