Put the cap back on.
I would agree you have a head gasket issue.im having same problem ,,ill be following,,,with cap off,i can crank it over without starting and coolant blows everywhere,, pretty sure head gasket
What ANTNY listed will work, but I use the gallon concentrate and mix it in one of the cheap pump sprayers.I think I will add that to my regular preventative maintenance...any other suggestions?