Pressure in coolant overflow tank?

In Gen 3, the cooling system does not have an "overflow tank", it has a "degas bottle". This is a newer design for cooling systems.

In older systems, there was a pressure cap on the radiator, and all of the system pressure stayed behind that, and the "overflow tank" was just a kind of stagnant backwater that would receive coolant that expanded past the radiator cap, and let it be slurped back in as the engine cooled. The "overflow tank" itself was always at atmospheric pressure.

On your Gen 3, you will notice there is no pressure cap on the radiator, there are multiple hoses connected to the "degas bottle", and the bottle's own cap is a pressure cap, marked 108 kPa.

The degas bottle is always under the same pressure as the rest of the cooling system, and if you look while the engine is warmed up and running, coolant is always flowing through it. That way, any bubbles in the cooling system get delivered to it, and remain in the top of the bottle (notice the "F" line is well below the top, leaving room for air). That's what makes it a "degas" bottle.

So right after engine shutdown, it is perfectly normal to have pressure there, even almost 108 kPa of pressure. (And yes, that's enough to pose a risk of hot coolant escaping to scald you, if you remove the cap then.)

Holding pressure for more than a day, that's weird.

Do you have any other engine symptoms that are concerning you?

What made you interested in opening this cap right after engine shutdown or a day later?

 

Gen 3 did away with the traditional radiator cap and overflow tank. The tank is a part of the coolant system and coolant circulates in it like the rest of the system, so it's under the same pressure as the rest of the system.

Edit to add that apparently @ChapmanF types way faster than I do. He also give great information.