So when I have my air on, either the heater or the AC, whenever I accelerate all the air goes through my defroster vents. Once I'm off the gas, the air will change back to whatever setting I had it on I.e. Panel, floor, panel&floor, etc. I can't seem to figure why this is happening. Any ideas on how to fix it or what causes it???

 

You had me curious so I pulled this from another f150 site for same issue:

the air doors inside the duct are controlled by vacuum cylinders. if you lose vacuum, the cylinders go to the default position, and air comes out the defrost. normal areas to lose vacuum are cracked hoses at the firewall under the hood, between glove box, and center of truck,,, and the vacuum canister outside of the battery, in the fender well. small crack in hose or TEE will cause the issue.

 

Google f150 oring EATC.

 

You have a vacuum leak. At idle or low throttle input, engine vacuum is as high as it gets because the throttle plate is closed and the engine is starving for air. In this condition a small leak may be overcome by the high vacuum. Once you open the throttle, vacuum is reduced and whatever leak you have becomes a problem. Keep in mind, this leak may not be part of the HVAC system but could also be under the hood.

 

I have the same problem with my 2002 F-150, but mine will jump from A/C to Defrost at any engine power setting. My mech also has told me that these trucks are notorious for vacuum leaks and he could do
a smoke test (???) to find it.

 

They use a machine to blow smoke through the vacuum hoses and see if it's leaking out of any of them. It helps to identify where the leak is if any.

 

So far all of these replies are useless, this is common in every f150, beside the battery and firewall, there are alot of hoses, 1 goes through the firewall, check that hose first, check the blue valve. And check all the other hoses in that area, it could also be a hose on the vacuum reservoir behind the battery and inside the fender

 

This is an old thread and the person who revived it didn't really ask any questions, just seemed uncertain what a smoke test was.

Also the earlier replies weren't useless, they led the person in the direction that it was a vacuum leak, which you've also just done...

 

And he said to check vacuum hoses for leaks... which is what the smoke test you explained will do too so really his post is useless

 

Well smoke tests require a several hundred dollar smoke machine, unless you make one, and they usually dont work on hvac lines as good like air conditioner, im tellin you check those lines around the battery for cracks and make sure the valves work, i guarantee thats where your problem is