My wife drives a 2001 F150 4.6L. It has developed a new and annoying habit that I think we've established a pattern for, but I thought I'd get a little help on.

The vent air seems to have a mind of it's own. Under light engine loading where vacuum is higher) all is fine... the A/C air comes out of the panel vents. However, when you have to push it a little harder.... the panel vents shut off and the cold air runs up to the defroster vents until you back off the throttle and allow the vacuum to come back up, when the air comes back down to the panel vents.

Just tonight I was trying to notice the finer points of the problem and actually had it under control (in a manner of speaking)to the point where I was cruising at about 55. If I pushed the pedal any harder... the air went up to the defrost vents, and if I would ease it back it would smoothly come right on back down to the panel vents.

This is the only thing that shows up....they are rock stable at all times. It is like something is supposed to be regulating vacuum somewhere..... and it isn't working right!

If someone has an idea I'd appreciate it. I have uncovered through some poking around, that this truck has a vacuum reservoir and also a vacuum check valve but I have no idea where they are located.

Thanks for any help you can give,
Bob

 

Check the vacuum tubing on the firewall in the battery area. One should run into the fenderwell. This one goes to the vacuum canister to keep it from doing that. It may be cracked somewhere.

 

sounds like the check valve is bad going to the canister.

 

https://www.ford-trucks.com/forums/6...ht=vacuum+hose