Bypassing the IAC in a Volvo 740

Discussion in 'Volvo 740' started by shrike, Apr 23, 2005.

  1. shrike

    shrike Guest


    My 740 is reliable transport, but the body is in bad enough shape that
    it really isn't a keeper. I've got an idle surge when cold but not when
    warm. I read the faq, and have cleaned the TB, replaced the flame trap,
    and sprayed about a half a can of carb cleaner through the IAC until it
    ran clear. The idle cleared up for a little while, but now is worse
    than it was.

    My issue here, is that at $175, I'd rather replace the vibration damper
    (which needs replacing) than the the IAC. I am looking for solid
    transportation, not a perfect car. My A/C doesn't work (and I have no
    plans on fixing it), so the only time the IAC is really needed is at
    startup. Convieniently I live in a state with weak emission standards.
    I was wondering, has anybody just bypassed this stupid thing?

    What would happen if I just stuck some tubing in place of it?

    shrike, Apr 23, 2005
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  2. In theory, you could probably replace the IAC with a manual valve (wide open
    tubing would run the idle over 2000 rpm). But I think you'd be happier with
    the valve the way it is than with that alternative.

    The IAC is a pretty simple device but when it gets gummed up it can be a
    headache. Pull it out again and use a wooden stick to rotate the vane inside
    it. If it doesn't move very freely, you need more carb cleaner! Give it a
    shot of cleaner and rotate the vane - repeat until it is at least moving
    smoothly (there will still be some drag). When you are done you should feel
    the vane clunk against the stops when you rotate the IAC valve in your hand

    The other thing that goes wrong with them is the O-ring where the motor body
    is crimped to the valve body leaks and lets oil into the commutator. It is
    possible to uncrimp the housing and clean it up, but it is easier to get an
    IAC valve from a wrecking yard.

    Michael Pardee, Apr 23, 2005
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  3. shrike

    shrike Guest

    Thanks Mike,

    What about routing it to the air cleaner? There is a nipple on the air
    cleaner box that looks like it was intended for a hose, but it is
    blocked. I was thinking I could reroute this IAC hose to the air
    cleaner and block off the port where the IAC connects up just before
    the throttle body. Would that give me a normalized idle without the

    Perhaps I am not understanding exactly what this bugger is doing. I
    mean I understand it recirculates crankcase gases, but is crankcase
    pressure really that high that it would change the idle to 2K RPM? Wow.
    Could the idle be dialed down from there to normal?

    My idle is surging bad enough where I have to start the car two or
    three times before it will idle normally. So it is fairly annoying, not
    to mention the extra stress on the starter.

    Considering the number of posts related to this problem previously, a
    boneyard IAC would seem a suboptimal solution. (Could get one just as
    bad). Not to mention that if this thing is controlled by the computer,
    it may actually be the computer or some sensor causing at least part of
    the problem. The wiring in this car is pretty well disintegrated. So I
    am not inclined to start troubleshooting sensors and wiring if you know
    what I mean. I'd just like the car to do it's job, with less fuss.

    -Thanks in advance
    shrike, Apr 23, 2005
  4. Actually, it is a bypass valve for the throttle plate. It is just a 1/4
    revolution motor, complete with commutator, rotor windings and permanent
    magnet field. If the ECU sees the idle sense switch on the throttle body
    making contact, it sends DC pulses, one polarity or the other, to increase
    or decrease the amount of air that gets past the throttle plate. When the
    throttle is opened a bit (or the sensor switch is unplugged) the ECU opens
    the IAC valve all the way.

    Yes, there is a chance an IAC valve from a wrecking yard will be dirty and
    even intermittent (if oil has gotten in the commutator). Your chances are
    decent, though.

    You mentioned in the original post that you cleaned the throttle body, but I
    was wondering if you did it on the engine or removed the throttle body. That
    passage at the bottom, where the knurled plastic idle adjustment screw is,
    loves to plug with deposits. Vacuum drags all sorts of stuff into it.
    Anyway, it is hard to clean without removing the throttle body. Once the TB
    is in your hands, you can count the number of turns the idle adjust screw is
    open, then remove the screw and blast cleaner through from each side of the
    throttle plate to the passage the screw came out of. If that passage is
    blocked, on either side, the IAC will overcontrol the idle and produce the
    sort of surging you describe.

    Michael Pardee, Apr 23, 2005
  5. shrike

    grtdane63 Guest

    Having rotten wire will give you thiose symptoms also, a wiring harness is
    not that bad of a job to install and you can pick up a used harness from
    Dave Bartons site for a good price. The link is

    grtdane63, Apr 24, 2005
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