Air conditioning problem on a F150

[PushinTheLimit]

My buddy has an F150 with a 5.4L triton motor and he is having a problem with the air conditioning in his truck. Its something that I've not seen before and I really dont know how to fix it. I wanna see if any of you guys have an idea about it.

With the air conditioning on full blast just sitting at an idle, the air flows thru the main vents just like normal. Even driving around with little throttle response, the air does just like it normally should. The problem comes when he starts to accelerate. When he accelerates the least little bit, the air flow will almost like bypass the main vents and start to come out at the defrost and at the feet. It will still blow cold air, but just changes where the air flow is going to. Has anyone ever experienced this before? I just have no idea of what to tell him it could be is that some way its bypassing the vents somehow.

The truck has about 115k miles on it right now. Anyone got any ideas?

 

[MikeW]

Humm a buddy of mine has it in his GT >.. http://www.svtoanashville.com/forums/calendar.php?do=getinfo&&day=2007-9-21;e=94&c=1

 

[Gallows]

What year F-150? Does it have electronic climate control?

 

[Zorabot]

Sounds like a loss of vacuum is allowing a door to close in the HVAC box changing the direction of the air, possibly a bad check valve or vacuum reservoir.
I dont think its the EATC as that uses electric motors to move the doors.

 

[CobraBen98]

Sounds like a loss of vacuum is allowing a door to close in the HVAC box changing the direction of the air, possibly a bad check valve or vacuum reservoir.
I dont think its the EATC as that uses electric motors to move the doors.

+1, I was going to say a vacuum leak somewhere. With that kind of mileage, it would just be best to replace all the vacuum lines at this point.