Coolant

Hello, I'm Danny.

If you say it's coming out from the radiator cap then it sounds like it may be a defective cap.Radiator caps have a specific p.s.i. rating and do fail like any other part. Cooling system pressure testers that are used to pressurize/test cooling systems do come with an adapter to test the cap. Here is a tutorial for you to view that shows what is involved:

https://www.2carpros.com/articles/radiator-pressure-test

Most local auto parts stores have a tool loaner program and can rent you a pressure tester.Hope this helps and thanks for using 2CarPros.
Danny-