AC mode changes at half throttle to Defroster mode. Leaving up on the throttle restores the air back to the dash registers.
Need some help.

1998 F150 XL 4.6L

 

You have a vacumn leak somewhere. When you step on gas the vacumn drops and causes the door modes to change.

I'd check the resevoir and the pcv line for starters.

 

Where are those?

 

There is a single vacuum line that runs through the firewall from a manifold near the intake. Locate it and check for cracked or split tubing.

Steve

 

Problem found

 

Problem found on the passenger side firewall. A black, spaghetti size, line running from a small vacuum check valve to a rubber grommet in to the firewall. The line was leaking about three inches back from the firewall due to abrasion. I took a piece of heat shrinkable tubing and slid it over the leaking line for a temporary fix. I’ll put a new line on later.
Thanks to all who replied.

 

^WTF?....reported