Loose Bolt on Intake

Yup. That's one of the bolts that holds the intake and exhaust manifolds together. My bet is that it's broken, that's why it won't tighten up. What does the end of the bolt look like? If it's broken, you're gonna have to remove the intake and exhaust manifolds, separate them remove the broken bolt and put it all back together. There's no vacuum leak there as the manifold has a tube the bolt runs through.