Air leaking through vents

[JD89Suburban]

I have a 1989 Suburban with a 5.7 with a/c. When driving down the road I can feel air blowing through the vents even though everything is off. When I am stopped there is no air flow. I can switch it from hot and cold and the air temp coming through will change but the fan isn't running. Where should I be looking for the air coming in? Should I start with resealing around the firewall? Thanks.

 

[1987 GMC Jimmy]

As far as I know, it's supposed to do that per GM's "Flow-Through Ventilation." Mine do this, and if it's not hot enough to run the air, which is rare, I'll just blend the temperature depending on ambient conditions.

 

[74 Shortbed]

Neither one of my 89 Burbs have air flowing through the vents when everything is shut off???..

 

[1987 GMC Jimmy]

I could be wrong, but I thought "VENT" was supposed to use the blower with outside air, and it was just supposed to trickle when it was off. If my cowling is clogged with junk, it'll prevent that.

 

[74 Shortbed]

Yes, if you put it on "Vent" it will flow air, but if you move the lever to off it stops, at least on mine they do anyways????..

 

[JD89Suburban]

Yes it is doing it when everything is off but only when I am moving at highway speed.

 

[74 Shortbed]

I'm not into this new stuff so I could be wrong, but my guess is the vent door is stuck partially open???..

 

[1987 GMC Jimmy]

Mine's the same way as yours. It's not doing anything not moving, but on the highway, you can feel it.

 

[74 Shortbed]

Only time I can feel it is when I move the lever over to "vent".

 

[JD89Suburban]

Guess I will be tearing apart the dash this weekend and looking into the vent door.

 

[bucket]

Isn't the door that controls outside air flow the one in the passenger kick panel? I'd look there before tearing everything out of the dash area.

 

[JD89Suburban]

On mine the door in the passenger kick panel opens for the recirculation on max a/c and there is a door under the wiper cowl that closes for the a/c but is open on all other settings. Would the air becoming through the cowl?

 

[RecklessWOT]

My 87 is the same. In the fall/winter I'm always playing with the temp lever

 

[1987 GMC Jimmy]

Aha! I knew I wasn't dreaming this up! This is from my car's owners manual, which is an '86 model. It has a 99% identical HVAC setup to a square body, and I think any of the truck manuals should have the same little drawing. I'm not a hundred percent on that, but I can check sometime tomorrow night if needed.

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[bucket]

Aha! I knew I wasn't dreaming this up! This is from my car's owners manual, which is an '86 model. It has a 99% identical HVAC setup to a square body, and I think any of the truck manuals should have the same little drawing. I'm not a hundred percent on that, but I can check sometime tomorrow night if needed.

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Yes, but it should not be a very noticeable amount of airflow. And it seems most of the air should come through the floor vents, not the dash vents where it is easily noticed. GM car and trucks still did this well into the 2000's. Maybe even the new ones, I don't know.

I've owned and driven way more than my share of GM products and yes, there should be a minor amount of airflow and yes, the temp can be adjusted even in the newer ones with electronic controls.

But there should not be a noticeable amount of air that comes through the dash vents.