Here's a question: isn't a vacuum the absence of air? Why a cannister?

Discussion in 'General Motoring' started by Jamie, Jul 21, 2006.

  1. Jamie

    Jamie Guest

    Here's my question: Why does a car need a vacuum cannister? Mine is
    broken, it's in a crappy location to access and I want to replace mine.
    So, I am planning on buying a vacuum cannister and mounting it in
    another location. But why do I need it?

    Right now I have the tube that goes to the cannister shut off. My vents
    work better, but I don't see the difference between tying off the end
    of the tube, or connecting it to a cannister.

    Vacuum = no air. Tying off = no air. Why do I need to contain 'no air'?
    If the cannister has a one-way valve to let air escape, why not just a
    check valve?

    So, there goes. What purpose does a cannister serve versus tying off
    the tube?

    Jamie
     
    Jamie, Jul 21, 2006
    #1
  2. Jamie

    mjc Guest

    Since you mention "vents", I assume you are talking about the vacuum
    reservoir in the lower dash. You are half right: this isn't needed to
    operate the ventilation controls *with the engine running*. What it does
    do is provide enough vacuum to operate the controls for a brief time
    when the engine is shut off, or when engine vacuum is especially low.
     
    mjc, Jul 21, 2006
    #2
  3. I'm guessing you have an older model. Our '85 765T has a vacuum canister,
    and I repaired a crack in it with JB Weld years ago... no trouble since
    then.

    The vacuum canister holds the vacuum motors that control the ventilation
    system in the selected position when the intake manifold isn't producing
    much vacuum - hard acceleration or hill climbs. If you have a turbo model
    there is also a vacuum pump to maintain the vacuum for long hill climbs, and
    that needs the canister so it isn't cycling on and off rapidly.

    Mike
     
    Michael Pardee, Jul 21, 2006
    #3
  4. Jamie

    Jamie Guest

    Sorry if I was vague. The cannister I am referring to on my 1987 740
    GLE non-turbo is buried under the front bumper of the car. I could
    barely even see the cannister by crawling under the car, so there was
    no way for me to assess the damage without apparantly unbolting the
    front bumper.

    There is a vacuum line that comes through the firewall to the engine
    where it is t-connected to a hose going through the intake manifold and
    another to the vacuum cannister.

    There is a check-valve on the hose from the intake manifold to the
    t-connector. Well, I disconnected the tube going to the cannister and I
    blew into it and sucked. There was free flowing air both ways, so I
    figured something at the cannister broke. So, I removed that tube from
    the t-connector and sealed off the end of the t-connector. With the
    tube connected to the broken cannister I had little air coming out the
    A/C vents. When I sealed it off, a LOT of air came out the A/C vents.

    I want to buy some generic cannister and mount this somewhere under the
    hood where I can easily access it. My only real question was do I
    actually need one.If so, how does a cannister store a "vacuum" when a
    vacuum is nothing?
     
    Jamie, Jul 21, 2006
    #4
  5. Jamie

    KLB Guest

    Wouldn't the canister be acting as a sort of ballast. Under negative
    pressure the canister would be to the left side of zero (vacuum) as opposed
    to zero (nothing)
     
    KLB, Jul 21, 2006
    #5
  6. Jamie

    Jamie Guest

    Now that's interesting. Are you saying that by sealing off the
    connector, I get zero pressure versus some cannister that actually
    would cause negative pressure, or sucking, or in other words 'create' a
    vacuum?

    I can sort of see that. My confusion was I read that the cannister
    'stores a vacuum' and this didn't compute. If the cannister is somehow
    enhancing a vacuum by maybe providing a volume of area that would
    somehow serve as a tool to create that 'left of zero' you speak of, I
    see where you are going.

    Thanks!
     
    Jamie, Jul 21, 2006
    #6
  7. Jamie

    James Sweet Guest


    Whether it's vacuum or air pressure is irrelevant, it may be confusing
    if you're making it too complicated but there's still pressure, it just
    is in the opposite direction. You can "store" vacuum just like you can
    store pressurized air, you're limited however to around -14 PSI. If you
    had a big evacuated can of "nothing" you could connect it to something
    else and draw a partial vacuum in that, just as you could connect a
    pressurized tank to something and pressurize it.

    If you tie off the vacuum cannister your vents will close when you stomp
    on the gas and the engine vacuum drops.
     
    James Sweet, Jul 21, 2006
    #7
  8. Jamie

    Jamie Guest

    Thanks, the picture is becoming clearer.

    Would it be fair to say that the main difference between tying off the
    tube, versus having a cannister, is that without the cannister you are
    limited only to the amount of vacuum stored in the tube. With the
    cannister, you have a greater volume of vacuum.

    When you stomp the gas, the vacuum in the tube is quickly depleted -
    versus having more vacuum in reserve in a cannister?
     
    Jamie, Jul 21, 2006
    #8
  9. Jamie

    James Sweet Guest

    Yes exactly, just as if you were trying to store pressurized air to
    actuate pneumatic solenoids, which is exactly how the vents are operated
    except vacuum is readily available from the engine so they use that.

    It really isn't hard to remove the bumper to get at the cannister, just
    a few bolts.
     
    James Sweet, Jul 21, 2006
    #9
  10. Jamie

    Jamie Guest

    Ok, I'll give another looksee. I would like to put my new pneumatic
    drill to use. It's a shame to have all that torque and power just sit
    there.
    ;-)


    JB
     
    Jamie, Jul 21, 2006
    #10
Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.