I have a 91 hatch with a 95 gsr swap. Everytime I hit around 4k rpm on the highway, my recycle interior air level flips open. Assuming it has something to do with a vacuum leak, I checked all of the engine bay vacuum lines with a propane torch. I could not find the leak at idle. Does anybody know what else could be causing this issue? Thank you in advance.

 

Could this issue be caused by a/c system being removed? Maybe its just the speed on the highway causing air to force it open?

 

The outside air pressure is opening up your recirculating vent. Mine does the same thing at any speed above 50 mph.
I can get it to stay closed sometimes if I have the fan blowing on maximum and I hold the handle for a few seconds.

Not sure if the issue is on the lever side of the cable or the vent, but the opening and closing after 25+ years of use probably wore down whatever the basic keeper mechanism was.

 

I guess I will just plug up the intake. Everytime it flies open my cab temp raises about 15 degrees. Its already hot enough down here.