surge tank cap

I'm having serious problems with my 96 grand am (V6 3.1L). I'm pretty sure I've got a cracked head or something like that because there are air bubbles in the radiator surge tank. That and overheating are the only symptoms so in hopes that it is a minor crack I put sodium silicate in to try and seal it. When I drove it afterwards it didn't heat up but still spewed out from the cap. When I bought the car (used) she didn't have a surge tank cap-a friend put one on so I'm not sure if it's the correct psi. My question is what psi is required for my car, if anyone knows...

16 psi.

Thank you so much, I couldn't find that info anywhere.