My 2001 F250SD 5.4L is doing something odd. When I have the A/C on and accelerate, it shifts from blowing out cold air from dash vents and goes to blowing lukewarm air out from the defrost vents on top. As soon as I decelerate or tap the brake pedal, it switches back to the dash vents and blows cold again. Even minor acceleration switches the air from blowing out the dash vents to blowing out the defroster vents.

Any suggestions or is this a known issue?

Thanks in advance.

 

Start looking for a vacuum leak.

 

See faqs (link in my signature below).

You probably have a "two-fer".

 

I have the same truck and had the same problem. Found the solution on this website. You have a vacuum leak. I replaced rotten hoses to 4wd hubs and had no luck. I finally found a vacuum diagram and ended up plugging a hose under the hood and that solved my problem. The flapper that controls where the air flow is vacuum driven and is located under the dash. Find that and track your vacuum lines backwards from there. From what I have seen a vacum leak anywhere can cause this problem.Hope that helps.

 

The part that controls the direction of air flow is under the glove box and the vacul lines trace back uner the hood on passenger side. Find a diagram online somewhere.

 

The heating/AC system uses vacuum. If there is insufficient vacuum, the system defaults to blowing out of the defrost vents as a safety measure.