will it hurt anything if the cat. check valve coming from the engine to the exhaust is welded/clamped shut? This just doesn't seem right to me

 

You are talking about the split air pipe?

The split air pipe supplies air to the 2nd stage in the main cat converter, and if you have a 84-88 non turbo, supplies pressure to activate the aux ports.

So if it is not connected, the 2nd stage of the cat is probably not cleaning the exhaust as it should and if you have a 84-88 non turbo your aux ports are probably not working (robbing you of mid and top end power).

 

I couldn't find a picture...but it comes off the lower passenger side of the motor fairly close to the firewall. Kinda looks like a mini old school oil can? then the long metal tube goes toward the exhaust. But it's welded shut. I have a '90 na.