N71 ISV Function.

[twoqu]

Whilst I have studied the various diagrams, I would still like some information on how it functions.
In its non-powered state it is still possible to blow through the ISV as it has a bypass channel on it. This supposedly allows the engine to idle, at a speed (unspecified) above normal, if the ISV suffers a failure.
When the rotary valve is moved slightly, then that channel is shut off and its much more difficult, but not impossible to blow air through. I guess thats effectively blocking both the bypass channel and the rotary valve.
Move the valve further and then it starts to allow air through the channel in the rotary valve, which of course is how the ecu controls the idle.
So on starting, I am assuming that the rotary valve should be fully open. Then as the car warms up it should progressively begin to close according to the temperature until it reaches a position consistant with flowing enough air for the specified hot idle.
The only other operating condition should be a larger opening if the aircon (if fitted) is operated to give the aircon boost to the idle speed.

What happens to the ISV above idle? Does it allow air through by fully opening or does it rotate the valve to shut both the channels off or go to its default state of the bypass channel?

[UrS4boy]

N71 Idle Speed Stabilization Valve (ISV or ISS Valve) Info

Not sure if you have seen the info at the link below nor I am sure why you need to know what happens when the engine is off idle (I presume the ISV is shut):

http://forums.quattroworld.com/s4s6/msgs/21853.phtml

One of the live labels from this Engine Bay Device Map:

http://members.shaw.ca/speedtek/UrS%20Engine%20Bay%20Device%20Map.html

[twoqu]

Indeed! Thats whay I said I have studied the diagrams!
I am interested in its behaviour.

[newsh]

I imagine the idle control is a closed loop. So if you're looking at a spare valve that is plugged into the loom then it will close right down trying to reduce the idle speed, if it's not actually in the idle circuit then the revs will stay up and the ecu will keep the valve closed. If you could somehow reduce the idle speed (in gear and slip the clutch?) then the ISV should open.

[twoqu]

I imagine the idle control is a closed loop. So if you're looking at a spare valve that is plugged into the loom then it will close right down trying to reduce the idle speed, if it's not actually in the idle circuit then the revs will stay up and the ecu will keep the valve closed. If you could somehow reduce the idle speed (in gear and slip the clutch?) then the ISV should open.

John,

I am trying to find out how the ISV should behave normally when connected and working correctly!

[newsh]

Yes, and I'm trying to help.

[twoqu]

Yes, and I'm trying to help.

Good! Can you pull your ISV connector off and tell me what rpm the car idles at!
If its 1050rpm we might be onto something.

[newsh]

Yes! With the connector off the idle immediately goes up to 1050rpm! Also if I try to open the throttle and hold it constant it hunts up and down.

[twoqu]

Yes! With the connector off the idle immediately goes up to 1050rpm! Also if I try to open the throttle and hold it constant it hunts up and down.

So would you agree thats the same symptoms as Lou is experiencing?
Ergo his ISV is not being controlled or it is in the default mode.
Thanks thats a big help.
Think we should have had this discussion in the other thread!

[newsh]

Yes, having driven his car this is exactly the symptoms..

Seems his ISV is effectively disconnected.

There's a wiring fault somewhere. I would test from the ECU connector to the ISV connector with a meter, for continuity and shorts.

[twoqu]

It could just as easily be the idle switch side. Of course if the ecu isn't told to idle it won't control the valve. Could even be a power supply issue.
We will have to try and systematically eliminate potential causes.

[newsh]

But I had the same off idle symptoms too when I disconnected the valve?

[twoqu]

Yep!
Basically we are pointing the finger at the ISV. We know how you duplicated the fault. But there are more possibilities for Lou's problems.
We will get it!