What's Going On Behind The Glovebox? (Vacuum Controls)

[Archaeor]

So I know there's quite a few vacuum controls behind the dash (climate control, heater, AC, etc.) I can hear them open and close when I start the truck, change heater/AC settings, etc.  Here's my question: When highway driving (and sometimes city driving) I have the interior fan off, Climate Control off, Heater off, both levers to the left.  Every so often, especially when cresting a hill, I hear a vacuum control move behind the glovebox.  Any idea what it is?  I'm puzzled.....

 

Thanks,

 

Rick

[cruiser54]

The load on the engine is causing the vacuum to drop. The mode door is moving to the Defrost position.

 

Reason? There is a leak in the vacuum system for the HVAC or the storage cannister is bad.

 

[Archaeor]

Thanks cruiser54.  How would I go about isolating the location of a the vacuum leak? I haven't seriously worked on cars since the '70s, but I seem to remember you can spray starter fluid? on the fittings and listen for a RPM change?

 

Rick

[cruiser54]

Chances are a visual inspection will reveal it. The tubing is very prone to cracking where it is routed behind the battery. 

I move the vacuum ball to the firewall and eliminate all those lines. 

[Archaeor]

Thanks.  Could this vacuum leak also cause my idle speed to rise from 750rpm (when cold) to 1000rpm when warm?

[cruiser54]

Thanks.  Could this vacuum leak also cause my idle speed to rise from 750rpm (when cold) to 1000rpm when warm?

Possibly.