Let me expand on aclmck's wisdom. You've got a pressurized coollant system. That means that the cap seals
the system with its spring and gasket
up to a certain pressure, which allows the coolant
to keep working above its base boiling temperature. If it gets hotter than that, some steam/coolant is released and makes a red mess. If the spring is weakened by age or the gasket
has hardened, it releases and makes a red mess at a lower temperature.
Failure of the cap in one of these modes is much more likely than failure of the neck that the cap sits on. It is also much cheaper than replacing the neck. So, make a cap failure your first hypothesis, inspect it, and even if it is just old, replace it with one marked for the same pressure before investing in less likely repairs
. Here's to a clean engine
room. I wish that I had one.