Given there's no air or other failures/problems, if the coolant comes out of an open and full radiator top when the Montero is started, does that mean exhaust gases must be burping into the coolant and pushing it out?

Or can it be something else, not related to head gasket?

One Montero I observed (with known head gasket failure) would keep pushing or burping coolant out past the initial start, about 1/2 cup per burp. Another Montero that hasn't been confirmed with a bad head gasket (no other signs) burps about 1 tablespoon of coolant at initial start and then doesn't do it again.

What does your Montero do? I'm wondering if this burping radiator coolant at startup is a valid sign of failing or failed head gasket or not.

 

I'm not too sure about that since my experience with these vehicles is on the low side. In general though, wouldn't the liquid consistency of the coolant be an obvious sign (outliers withstanding)?