Bypassing the IAC in a Volvo 740

Howdy,

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?

-Thanks
-Matt

 

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
rapidly.

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.

Mike

 

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
IAC?

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
-Matt

 

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.

Mike

 

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
http://personal.linkline.com/dbarton/

Harold