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Blowing air with HVAC off while driving

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by 2003-5.3-V8, Nov 6, 2012.

  1. 2003-5.3-V8

    2003-5.3-V8 New Member

    I'm having a bit of trouble finding out what this problem is.

    when I turn the A/C and Heater off the air stops blowing while the vehicle is stopped (as it normally should)
    But when I start to accelerate and while constant speeds, air starts blowing out the bottom floor vents, and what ever temp I had it set on before I turned it off is the temp that blows out the bottom vents.

    Anyone know what the problem might be?
  2. WorthFlorida

    WorthFlorida Member 2 Years 100 Posts

    I assume it is an electronic control system with a digital display. It sounds like the outside door vents are not closing so as you drive the air is forced into the vehicle by the front air pressure. It may be a vacuum leak to one of the vacuum motors that closes and opens the vent/defrost/heater vents. They used to be vacuum control but they may be electric motors, I do not know. First check all vacuum hoses under the hood. It will usually be a very small hose (like the washer fluid hoses). That age of the truck those hoses get brittle and leak. Look under the dash too. The other question is in what position should the vacuum door close in. I would assume in a normal situation they would go the a/c vents. This happens on all the vehicles I ever owned so there is always fresh air flowing into the passenger compartment at all times. When switching from heat to a/c to defrost, does the air flow change? What happens when you shut the system off in defrost mode?

    PS I assume that the fan motor does turn off.
  3. j cat

    j cat Rockstar 4 Years 1000 Posts

    with the vehicle moving air will flow into the cab blower off . this is a design that allows some air to enter the cab with the vehicle being operated to reduce the possiblity of CO poisioning.

    looking at the rear of the cab on each side you will see vents. these are the exhaust vents. which is part of this design .

    all GM vehicles I have owned have this system.

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