Vents changing with engine load

[tekrsq]

I have this cross posted in hopes of more people seeing my issue.

While sitting perfectly still, the front heat blows just fine, and I can change the vent positions according to the EATC screen. However, while driving, if I put the engine under load (accelerating, going up a hill, etc) the vents will switch to the defrost position. It doesn't matter if I'm running heat, AC, or whatever. If I let off the gas, the vents will return to where they are supposed to be. I'm assuming this is a vacuum issue, but I cannot find any leaks. I've sprayed every hose under the hood I can find with carb cleaner, and there have not been any idle changes. So can somebody give me a general idea where to start looking in greater detail ? At some point this summer, I'm going to have to replace all the vacuum lines. They are seriously dry rotted....but for now, I NEED heat.

FWIW. Both heater cores, heater hoses, blend door, and blend door actuator were replaced about 4 yrs ago.

 

[tekrsq]

Nevermind. Looking through a bunch of posts with the same issue, I discovered it's probably the vac line coming off the vacuum reservoir. So I'll start trying to find that.

 

[ExplorerTom]

Look behind the battery.

 

[wpatters1229]

I had the same problem and there is a thin vacuum line behind the parts on the left side of the firewall. Replaced with a piece of hose and everything now works.

 

[tekrsq]

I had the same problem and there is a thin vacuum line behind the parts on the left side of the firewall. Replaced with a piece of hose and everything now works.

Are you calling the driver or passenger side as "left side" ? Inside or outside ? There is a series of small vacuum lines on the exterior, passenger side fire wall behind a bunch of crap. I've replaced most of that mess with actual rubber lines instead of the hard plastic crap Ford used. It helped, but didn't completely sure the problem. I'm almost positive that whatever the issue is behind the dash. I just haven't time to pull the dash out and see what's going on back there.