Air/heat blowing some times on a chevy/GMC ??

[Oceantrav]

Have a 2007 GMC savana 3500.

Lately I'll get in an turn the heat or air on and nothing comes out, well it barely does. Then the faster your drive the more it comes out. Then I'll park it and go in the store and come back and it blows out strong and perfect.

Lately it's been working like 1/3 of the times only.

What could be the cause of this?

 

[NYCO]

I have a 2005 savana and have had something similar, but might not be the same. Occasionally I had little to no air/heat come out. I heard a slight hiss and noticed some hoses hanging down under the passenger side dash in front of where your legs would be. They are vacuum hoses or whatever and I had one tee that had come undone. I imagine there are multiple locations that you could check though. I also don't know anything about cars and someone else probably has better troubleshooting, but that did fix it for me.

 

[Teamoatmealpie]

Check to see what get on each fan speed. I have a fan relay going bad, when its not acting right I only get the middle 2 settings to work, and high and low dont do anything. The other 75% of the time everything works fine.

if you google fan relays for chevy there are a fair number of links. Your issue could be something else, but worth checking it out.


https://www.youtube.com/watch?v=rLeXeb8SEA0

 

[Buddha.]

If the blower resister is bad you wont get the slower speeds but you'll still get high I think. If the vacuum system is bad I think it defaults to defrost.