Welcome back:
What I need you to do is first confirm the cooling fans are running with the AC on. Next, we need to check and confirm there is no head gasket issue. Here is a link that explains how to check for a bad head gasket. The portion I want you to focus on is the one where it shows and talks about air / bubbles getting into the coolant. It's actually step 3. However, you really should look through all of the steps.
https://www.2carpros.com/articles/head-gasket-blown-test
Let me know what you find. Also, you mentioned the gauge starts to go up with the AC on, how high does it get?
Joe
Thursday, August 15th, 2019 AT 10:37 AM