Overheating due to radiator cap

[Claudio]

Hello everyone!

I'm new here but I've had a Yaris sol 2007 since 2013. I can say that is one if the most reliable cars out there.

So, it's very weird when the car doesn't work and a couple of weeks ago my car overheated and turned itself off. First time this happens and what was even weirder to me was that when I popped up the hood the coolant was splashed all over the engine, the liquid had burst out through the overflow tank.

I checked to see if there were any leaks but nothing, at the moment I didn't check the radiator cap because there wasn't any smoke or air coming out of it.

I waited until the engine cooled off and put some more coolant. I drove some minutes and the overheating light came up in the dashboard so I pulled over again. Eventually, I made it home but it got me wondering about the failure.

I didn't move the car anymore and days later I checked the cap and noticed that the center/middle part of it was missing so, I infer that the overheating was caused by the faulty cap.

My question to you guys is, should I be worried about the middle part missing? Should I open it up to see if I find it?
And how should I refill the coolant, directly to the radiator ?