Yep a no pressure cap is necessary for sure. Another way to check exhaust gas in cooling system is to find a garage with an old emission machine. You can put the exhaust sniffer at the top of rad w/o cap on and detect hydrocarbons...… If they are present, head gasget issue, if not, you're in the clear. Well in the clear as far as a tear down, but be diligent to locate the issue and service it.
