A/C ducts during idle

[6.2 G-30 van]

Hello there. At idle speed only, the air conditioning stops blowing out of the dash vents and only blows out the defroster and floor vents. Once I resume driving, the cold air blows out of the dashboard ducts again. I removed the dashboard but all the A/C hoses seemed fine to me and were connected. Does anyone know what's wrong? Thank you in advance for any replies.

Frank.

1993 Chevy G-30 van 6.2 naturally aspired

[DmaxMaverick]

Sounds like your vacuum pump is failing, or you have a significant vacuum leak somewhere. A significant leak can cause the pump to fall behind at idle, and keep up at higher RPM's.

Check your vacuum pump performance right off the pump. You should get at least 15" at idle with ~22" being optimal. If the pump checks out, start chasing the lines for leaks.

[diesel65]

You need to check your vacuum lines in the engine compartment, make sure everything is connected and no leaks. If everything is ok,then your vacuum pump most likely is failing. Connect a vacuum guage to the pump directly and you should read over 20 inches of vacuum at idle.

[Subzilla]

My Blazer does the exact same thing. I haven't taken the time to diagnose it, yet. I always tell people that my computer has sensed hotter air toward the bottom of the cab and is making an automatic adjustment to compensate for the difference. As soon as the lower quadrant has been stabilized, the computer will redirect the cold air back to the upper quadrant. It's amazing what technology they had back in the eighties! (It seems there's too much hot air in the driver's seat).

[ZZ]

I just started enjoying my electronic unit a little more. It stays on the vents.