Thanks for this video! Just another thought. My 98 Mustang GT is stored, but I was doing some work on it today replacing dash bulbs and pulled the stereo and CD player (all original) out and had the exact same problem when I went to try the fan it would only blow defrost. I suspected this, but had to start the vehicle to put my OCD to rest.. LOL When I was going through the settings checking everything after being in the dash, I exhausted all vacuum in the vacuum can and had no vacuum to open and close the doors to reroute air. So, just so I could sleep tonight I went out and started the vehicle built up some vacuum and then everything worked as usual. So I hadn't knocked a line off, I just exhausted all the vacuum. Give that a thought if you're seeing this in a stored vehicle or one that hasn't been started in a while.
I had the same problem. I had to fix the vacuume lines behind the glove box where there is a small tank with lines coming off of it. Make sure all the lines are connected.
I put my 04 mustang GT automatic dark shadow gray in the shop a year ago to have valve cover gasket put on and after the job was completed I was driving home and turn the air on and no air was blowing in my face so I turn around and went back to the shop pep boys and they looked at it and told me I needed 1500 dollars of work done and I had to leave the car overnight and I refuse carried my car home and it took me nearly 5 hours to find that cut in that small hose so I went to the junk yard and bought a hand full of those little hoses and piece it together and I have a bunch of it so I will never ever let pep boys work on my car again old mechanic trick to make money
I have the same Make and model and same problem and found the part of the tube that comes out of the firewall but can't seem to find where the other half is. It's like the hose isn't there at all.
It took me literally 15 times looking. The hose had fallen underneath something. It may be easier to just buy a new air hose and run it off the vacuum tree.
It also could have just broken completely off. After talking to several others, this was the case. They had to replace theirs with a new complete hose. Good part, it's only a couple dollars to buy new!
I have a 99 six - auto - mine does this occasionally - bought the a/c check valve, but can't find exactly where it goes - can't find vacuum diagram for my car - not in my repair books -
Go here and just look through the pictures! There are a ton of them. Just make sure you are looking at the right year! you can also look on FIXYA.com (Copy and paste the link) www.google.com/search?q=1998+ford+mustang+vacuum+line+diagram&sxsrf=ALiCzsYTmgQ0LoEmilO2AKteKJGGMD9xkw%3A1661870308877&source=hp&ei=5CAOY8TtMs31gQaYxYaIDw&iflsig=AJiK0e8AAAAAYw4u9KKH9qVdQxw7TO2GCHMYgSaGohQi&oq=1998+mustang+vacuum+line+d&gs_lcp=Cgdnd3Mtd2l6EAEYATIGCAAQHhAWMgYIABAeEBYyBggAEB4QFjIFCAAQhgM6BwgjEOoCECc6CAgAELEDEIMBOggIABCABBCxAzoICC4QgAQQsQM6CwguELEDEIMBENQCOg4ILhCABBCxAxCDARDUAjoLCAAQgAQQsQMQgwE6BQgAEIAEOgUILhCABDoICC4QgAQQ1AI6BwguELEDEAo6BwguENQCEAo6BAguEAo6BAgAEAo6CAgAEB4QDxAWUOQDWPRQYNmRAWgCcAB4A4ABigWIAYonkgEMOS4xNC4xLjAuMS4zmAEAoAEBsAEK&sclient=gws-wiz#imgrc=WQYIoxP0XT8ebM
I have the same issue with my AC and the car is not cooling since cold air doesn't circulate properly. In my city temp is 40 C all most days, I'm definitely trying this tomorrow and see if this fixes it.
Perhaps the hose doesn't have vacuum? Or check to see if there is a kink. I heard there was a check valve on the same line, but I have not looked for it. All three would cause this to happen. If it is a mechanical issue, you can try removing the faceplate of the air handler and see if the knob is still connected. There are several small gauge rods (they look more like bicycle brake cables) that physically move the air handler.
