say fellas, i have removed this part of my 68 with factory air for inspection. all is well believe it or not,but the vaccum hose that works the small door is missing. i'm not about to go digging under the dash again to try to find it and route it back so here's the question. should i leave the plenum door closed permenantly or prop it open?which is better for cooling while the air is used on high almost all the time.(it's hot a hell in the summer here).i don't know about the plenum under the right windshield area because i haven't gotten to it yet. i know the vaccum is inoperable there too so what's best to do with it? thanks for any advice as always.

Remove the spring on the door and push it open. If you are talking about the pass kick panel valve then what i said above is right.

yes i'm talking about the plenum under the passenger side kick panel .excuse my density but should the door of the plenum stay open all the time,or should it ever close? i can prop it open if necessary for better air flow. thank for helping.

What you want the door to do varies...

If you want A/C with recirculated air you will need the kick panel open and cowl closed.

If your cowl is open... then an open kick panel will generate a significant amount of raw air flow into the cabin.

If your cowl is closed... then it does not matter much and you can just leave the kick panel open.

so basically just leave the kick panel plenum door open and be done with it?sorry for all the questions but i'm not clear on it's function.thanks a lot

In a non-AC car the kick panels are simple vents and open up into the cowl plenum. Since the plenum intake is in a high pressure area, those vents are capable of providing a considerable amount of air flow. Especially given an exhaust like an open wind wing.

When the AC system was added GM came up with a rather creative way to provide a recirculated air flow. They would close the right side cowl plenum off from the intake using the upper cowl flap. Then they would open up the kick panel vent to provide an alternate air intake. This works because the blower fan pulls it's air from that side of the plenum.