A friends 2000 Monte with a 3.4 v6 suddenly started running rough while blowing a lot of coolant vapor through the exhaust and the temp ran up. Fortunately he quit driving it and had it towed home. It will run but of course it blows out vapor. It appears that it has lost very little coolant. Before I start testing for a bad head gasket I wanted to know if there is any coolant connection or gasket that could allow coolant into the intake that could be causing this. Thanks if anyone has any idea on this.

I'd consider doing a rad pressure test to see if it holds pressure.
Upper intake manifold could be the issue.Is there any oil getting into the coolant reservoir?
Head gasket,any bubbles or milky substance on dipstick?

I''ll have to look closer to make sure on the oil in coolant. No coolant into the oil, still medium brown color. There are some coolant passages that are within the intake that could leak into the intake passages through a gasket or crack?

I'd do the heads and intake.Replace O2 sensors and possibly the cats.