Ford HVAC Air Flow Issue

[Falcon_LS]

This is a bit of a weird one. 2001 Grand Marquis with manual HVAC controls; in any of the positions where air should be directed through the air vents in the dashboard (Vent, Norm AC or Max AC), it starts off in defrost mode, directing air flow to the windshield. If this was a constant vacuum leak, it would default to defrost, but after some time goes by and the engine is at idle speed with high vacuum, it will switch to the dashboard air vents all on its own. This one's left me scratching my head.

It also seemed to have happened round about the same time as my vacuum-assisted automatic parking brake release stopped working, so I'm wondering if the two are connected, considering they both use vacuum to function.

 

[Lubener]

I still suspect a vacuum leak somewhere.

 

[Falcon_LS]

Probably is, I'm going to have to hook it up to my smoke machine when I get the chance.

 

[tasch72]

On Ford vehicles of that generation with vacuum HVAC, if the system loses vacuum to operate, the failsafe position is defrost so that in case of emergency, you'll always be able to clear your windshield. It sounds like you have a split in a line. I would check the line that goes from your intake tract to your firewall first.