Air leaking through vents

[74 Shortbed]

Well, went for a ride yesterday and even at 75mph I did not feel any air coming out the vents...

 

[1987 GMC Jimmy]

According to the picture, it's showing that air should be coming from the top vents, the lap vents, and the floor vents. I would assume that a square body should get some in the face and the floor since no lap vents. You're right about the air volume. I'm not saying that it should be blowing your hat off. Driving 80-90 mph, the air coming in feels like slightly less than if the fan was on "LO," and the slower you go from there, the less you feel it.

 

[Rusty Nail]

Oh you're definately gettin it in the face...lol.

Move the selector to A/C and see what happens.

 

[74 Shortbed]

According to the picture, it's showing that air should be coming from the top vents, the lap vents, and the floor vents. I would assume that a square body should get some in the face and the floor since no lap vents. You're right about the air volume. I'm not saying that it should be blowing your hat off. Driving 80-90 mph, the air coming in feels like slightly less than if the fan was on "LO," and the slower you go from there, the less you feel it.

Hmmm, talking about the top vents and forgot about the floor, duhhhh , gotta go into town tomorrow so I'll see what's up with that..

 

[JD89Suburban]

Well after pulling the all vents off and checking all the vents except the blend door all are opening and closing as they should. So I started prepping to pull the air box off the firewall I noticed some cracking in the seal around the outside box. So I duct taped the seam and went for a drive. Well that stopped all the air flow. Now I just need to pull it and reseal it. Thanks for all the suggestions l.

 

[bucket]

According to the picture, it's showing that air should be coming from the top vents, the lap vents, and the floor vents. I would assume that a square body should get some in the face and the floor since no lap vents. You're right about the air volume. I'm not saying that it should be blowing your hat off. Driving 80-90 mph, the air coming in feels like slightly less than if the fan was on "LO," and the slower you go from there, the less you feel it.

It does show that and they should kinda sorta have a slight amount of airflow. Like if you lick the back of your hand and put it up to the vents you might be able to detect it. The floor vents normally have a strong enough flow to lightly feel it by hand. GM's goal was ventilation, not drafty and annoying. Of all my trucks, only one had fresh air noticeably blowing in the vents, but it had a constant dash vent door problem.

Well after pulling the all vents off and checking all the vents except the blend door all are opening and closing as they should. So I started prepping to pull the air box off the firewall I noticed some cracking in the seal around the outside box. So I duct taped the seam and went for a drive. Well that stopped all the air flow. Now I just need to pull it and reseal it. Thanks for all the suggestions l.

Glad you found the problem!

 

[74 Shortbed]

Another problem solved.

 

[RecklessWOT]

Well after pulling the all vents off and checking all the vents except the blend door all are opening and closing as they should. So I started prepping to pull the air box off the firewall I noticed some cracking in the seal around the outside box. So I duct taped the seam and went for a drive. Well that stopped all the air flow. Now I just need to pull it and reseal it. Thanks for all the suggestions l.

Man I gotta check that on mine. I know I have a blend issue, but sometimes I feel like there's too much air coming, dries my eyes out. I bet its something stupid. Coming from the desert this truck was awesome shape rust-wise (and still is) but everything ruber or foam (window seals, dash, etc) was hard and cracked to ****. I replaced as much as I could but never thought about the air box...

 

[Triv]

Another thing to check is the passenger side vent that only opens on Max A/C. If it don't seal well, you will get an excess amount of air flowing through the cab.

 

[chengny]

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

Sounds like this has been resolved, but it might be helpful to someone else - who notices changes in the ventilation system that seem to be related to engine load:


When available at the manifold, the vacuum is primarily used to stroke (and also hold in the selected position) the various diaphragm operated dampers associated with the air handling portion of the HVAC system.

When vacuum is unavailable at the manifold (or whenever manifold pressure rises above the pressure in the system/reservoir tank), any air flow will tend to be towards the system/reserve tank. It is at that point that the check valve (and reserve tank) come into play.

Due to the air flowing in the wrong direction, the check valve will snap shut. This effectively isolates the control system from the higher pressure within the manifold. With the primary source of vacuum no longer available, negative pressure for system operation is temporarily supplied by the vacuum stored within the reservoir.

Since there is essentially no ingress of air from atmosphere - in a well maintained HVAC control system - other than when the dampers are being moved, there is little to no airflow into the vacuum tank from the control panel. And, if the check valve is tight, there will be no flow of air from the manifold into the system. Consequently, the reserve tank can maintain system vacuum for a considerable amount of time.

For the most part, loss of vacuum in the manifold is the result of a WOT condition caused by heavy acceleration. But the vehicle can't be accelerated forever, at some point the driver lets off the gas pedal and the throttle plates are trimmed in the closed direction. With the engine still pumping air - and the plates causing a restriction to airflow into the manifold - pressure quickly falls below what remains in the system. The check valve opens and allows manifold vacuum to begin evacuating any air from the control system and re-establish the reserve vacuum in the tank.