Radiator cap still pressurized even after sitting for a month.

[jimtash]

Here's something that's been irking me and I'm looking for advice. What would cause the radiator cap to hold pressure even after sitting for a month while the hoses themselves are depressurized? I had to replace a leaking thermostat gasket and when I pulled the cap off to drain the radiator, it was still under pressure and coolant came spewing out. Is it the cap itself or something else? Radiator blockage somewhere? The coolant system works fine otherwise.

[JZRIV]

As we know heat is what causes pressure in the cooling system. As the coolant gets hot it expands creating pressure in the system. As the engine cools down, the coolant contracts reducing the pressure. If your radiator is filled to the top when its cool, a slight increase in ambient temperature is enough to raise the pressure enough to give a little spurt when the cap is opened.

This is more common on systems that had or have an overflow bottle added to it. With an overflow system the radiator is always full to the top since the coolant level is maintained in the bottle. I never had a 63 but on most older radiators without an overflow system, usually there is a "Cold Fill" line that is 2-5" below the cap level. This allows for expansion of the fluid. It may be as simple as your system is over filled when cold.

[CTX-SLPR]

Thermostat housing is below the fill level on the nailhead. You pull the housing or the hose and it'll gush coolant till the level equalizes.

[jimtash]

I think Jason nailed it down for me. I filled the radiator to the top a few months prior and that's what caused it to spill out the neck when I took the cap off.

Thanks.