1994 F150 Supercab.
For about a year and a half now, my heater has had this wierd thing going on. When the heater is on, and my speed is below 40-45 everything is ok. When I get up over 45, I hear a sound like a diverter flap closing and the temp of the air coming out of the heater goes way down, almost like drawing in straight outside air.
If I take my foot off the gas, I'll hear the diverter flap again, and the heated air starts coming out again.
Seems like maybe a vacume line issue that is causing an interanl air diverter to flip back and forth?
Anyone else ever had something like this, and if so, how were you able to fix it?

signed,,,
perplexed and cold

 

Sounds like your loosing vacuum under load and the blend door is switching to the default defrost setting. Under the hood on the passenger side is this actuator:



Check the white vacuum line like shown in the image. If its cracked or split, fix it.

 

Evan,
You're just full of help tonight.
I'll be checking that line in the morning too.

Thanks again,,,,
Chuck

 

That's what this pace is all about.

 

I would have said the same thing. This is a common problem.