I have a 94 F150 4x4, I have just noticed that whatever setting I have the air/heat on that the air will blow out defrost for a while then at the floor and then out the dash. I can have it on any setting and will do this. I checked the vacuum lines at the ac/heat switch but wandering if anyone has had this happen and what was the fix.
thanks in advance for any help.

 

I had the same problem with my Ford Taurus. It was the air conditioning check valve. Its located right under the hood on the firewall right in the middle. ITs a small part. Real easy to fix. I don;t know if the trucks have this valve but my taurus did.

 

 

Did you do what witt said? Did you fix the problem?
I still seams to me like it’s a vacuum leak somewhere, follow every line. And maybe a line cracked somewhere, or a line is rubbing something and made hole in it.
There are many possibilities; follow every line and I’ll bet you’ll find it.

 

My 1993 does the same thing. When you first turn the vent on it will blow out the defroster vents and then work okay. It's done it forever. I'm semi-convinced it's normal.

If it won't stay in the vents after it moves from vent to vent, then you've got a problem.

'93 F-150 loaded and modified!

http://briefcase.yahoo.com/toydestroyer1

 

Haven't had time to check been working late and it has been raining last few days and looks like more. I will check Saturday and post results. Thanks for the help!!!!

 

I couldn't find any bad vacuum lines or controls. I checked everything and for some reason seems to be working fine for now.