Heat fading in/out with engine load

[tekrsq]

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 sriously 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.

99 EB 5.4

 

[tekrsq]

BUMP !!!!!

 

[plumcolr]

Check the vacuum lines between manifold and vacuum reservoir, also the lines going to the front wheels if you have IWE (4wd). You have a broken line in there somewhere.

 

[00XTL]

Yes check all the vacuum lines in the engine bay, but they also continue inside the cabin behind the dash. Ive seen them crack at the rubber grommet where they feed into the fire wall.