Bypassing the IAC in a Volvo 740

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