This has been happening since I got the car, and I've had no luck figuring it out. When I'm driving at road speeds, there is always a steady stream of air coming out of the vents. usually is the main vents, even when the heater/blower motor is off. If I'm parked at a light or not moving, there is no air. It seems like it is being forced in somehow when I'm moving and I'm pretty sure it not supposed to be lol.
The latest thing I've tried is to just cap off the vacuum lines going to the HVAC controls(thinking that maybe a vacuum leak of some kind could cause something to be stuck open) but all this did was cause the air to blow through the defroster. otherwise it's the same. steady stream of air coming out when the car is moving.
Are there any weak points that could have given out that would allow outside air or engine compartment air to be forced into the heater ducts??

 

maybe you have the vent on?

 

nope. heater controls is set to "off". I've also unplugged the blower motor at 1 point just to make absolutely sure there's no short in it that's making it run. still blowing air when driving.

 

That is normal. The movement of the car forces air into the car, and the car has vants to let the air out

 

It's normal if the fresh air intake is open, but I don't think it's supposed to be with the switch off. The fresh air door is on the far right under the dash, behind the heater ac box. It's a vacuum operated flap on the hole that comes from the wiper tray to the heater ac box. I think you can access the the vacuum line to it from under the dash with the hush panel removed so you can test it. It's not a huge deal, but it does make the heat and ac work better when first in use.

 

 

Normal.............A offshoot of the old GM "astro-ventilation" years back.

Ever look at a 71-76 full size GM car trunk lid? Note the vent grid stamped into it.
From Wilki:
1971 models offered the same lineup as previous years including New standard features included power front disc brakes and inside hood release and a revised Astro Ventilation system that utilized air distribution grills in the trunklid

 

Don't think it's a vacuum issue because I unhooked the HVAC vacuum line, and the air still blowed out, just from defroster instead. Is that flap you're talking about the one you can see behind the blower motor when you pull it? I've actually tested that, it opens and closes like it should using the switch. is there a seal on there that can wear out or something? Are there any other openings that air could get into the vents from?

 

Huh. that's an odd design choice then lol. Still though... I don't remember my other 'bird ever doing this. It's really not even a big deal one way or the other, just kind of annoying smelling engine compartment air, especially now that it's getting colder and I've got the windows up more.

 

I don't see how this is relevant to a third gen

 

The fresh air flap is the only intake, except when it is in recirculated mode which I think uses the defroster as an intake. It should always go to defroster mode when you unhook the vacuum, but I can't remember the default for the intake. I'll check my service manual in the morning and see. The intake has a separate control vacuum tube to it, could still be a leak there if everything else is ok.

 

ALL GMs will have air entry thru the HVAC system to the interior
unless the a/c, def or recirc modes are selected or when the system is off.
My 07 truck and 11 Caddy along with my 84 Z have the same "draft"
while moving...............

This system is a ancestor/offshoot of the GM tradename "astro ventilation" system of the 70s. Thus its relevant.

 

Ok, it's sort of relevant, but also kinda wrong.
I wouldn't ever make an argument by saying anything is exactly the same in GM cars throughout the years and models, but here's where you are right and wrong.
The astro ventilation system is somewhat similar to the third gen cable controlled, non ac cars in that they have "ram air inlets" (fsm quote) that go directly to the cab for ventilation, similar to the astro except that also had extractors to improve it. Iirc, this direct air system was removed from many cars because of complaints of drafts and exhaust smell, but still used in others. I don't think any modern GM car has direct air intake.
After looking at the fsm I find you are correct that the default setting for ac system third gens is fresh air, but on third gens recirc is applied only for max ac. However, still very different from astro in that the air still has to go through the ac box, not straight to the cab.
Where it gets wierd, I think this is gms response to the complaints of drafts, if the mode selector is off, outside air is open and flowing through the box, but it is supposed to be applying vac to the defroster actuator to direct the non powered air there. So if that is not happening for some reason, you would feel it coming through the vents.

So to sum up, normal to have air flow in the box, but it's supposed to only go out defrost.
Not a huge deal, but I think it might indicate possible leak at the vacuum selector which could cause other problems, common failure (search hissing dash)

 

Thanks that's actually a thought I've been having(problems with the vacuum selector). I think I might have a vacuum leak somewhere and I haven't been having any luck finding it at all. I don't hear any hissing though... but my selector seems to select the wrong modes. like I put it on a/c and it blows out the floor, but put it on heater and it blows out the main vents. just weird stuff. I'll look into that switch at some point soon then.

 

Assuming someone didn't mix up the vacuum leads, that sounds like the selector unit failing. In addition to leaking they can also fail where the pieces get misaligned.
Does it still select all zones? Still select in the same order, just not matched to the indicator lever?

 

Check for holes in the floor or firewall. :-)