My 2001 Expedition has suddenly started blowing through the defrost vents with a very slight blow through the panel vents - when in panel, AC or Max AC. The thing that seems to be different from the other similar posts is that the other HVAC modes seem to work fine - floor, defrost and temp controls (blows cold or hot air as requested) all work as expected - only the positions that should be sending panel air do not work correctly.

I have searched under the hood for a vac leak to no avail - and can draw a vac on all lines except the one going through the firewall.

I'm not quite sure where to look in the dash, or how to get it open, which seems to be the next step. It has manual switches, not electronic temp controls.

And suggestions or links to similar posts or FAQS I may have missed appreciated.

 

That's the one that has the leak.

Drop the glovebox and trace the line inside the cabin. On some versions, there's a check valve or a reservoir, both have been known to break and leak vacuum.

 

Thanks projectSH089!

Everything looked ok behind the glove box but on checking the lines behind the dash on the drivers side ( sort of up and to the right of the accelerator pedal), found a vacuum line hanging free. Plugged it back in and panel air blows as good as ever!

Thanks for the insight!

 

Any idea why it was disconnected and hanging?

 

No clue at all! It was in an area that would be impossible to get a foot up into, so no way it was kicked off. Haven't had anything worked on behind the dash so it could not have accidentally pulled off while working on something else - it appears that the line just fell off, or maybe vibrated off but it fit tight when slid back into position so I have no clue....

It was working when we pulled into the drive the night before and did not work the next morning.