I have a 1994 F-150 and have noticed when going up hill or on the throttle when my A/C is on it would stop coming out of the vent and go to the defroster, then I would let off the fuel pedal and the A/C would go back to the vent, I have also noticed when the engine is running and the A/C is off I can hear a hissing sound behind the A/C controls. Could there be a vacuum leak and would the A/C control cause this to happen.


Thanks
Mudduk175

 

That is a sure sign of a leak,,, and sounds like you have it pinpointed.

 

I have been having the same problem with my 98 F-150! Like you said, when you let off the gas, or for mine, when you hit the brakes or in park, the A/C will come out the vents, but otherwise it is always out the defrost. It has been driving me nuts! Today is the first day I have used the board, and already I've found help! Thank you!

Laurel and MUDSLYD (my truck)

 

I am having the same problem on a '97 Expedition. Was it a vacuum leak? If so, are there any common problem areas I should check first? Thanks in advance.

 

Most common place to look is between the battery and the firewall. Battery acid gets on the vaccuum tubes and eats them up.
Also on 4X4's, the solenoids on the firewall by the battery also go bad and you can loose vaccuum there. You can check those by unplugging them, then cap the vaccuum lines and drive the truck and see if the problem is gone.