Revs hanging on

[UrS4boy]

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

...No problem with stalling so far but the revs do kick up quite a lot from idle if say the fans cut in, before settling back down again after a second or two. I've always wondered if that was normal or could that be the ISV as well?

I think that is a description of a dirty ISV that isn't able to react as it should to control signals sent to it by the ECU. Cleaning time.

Here is some ISV info:

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

[mcandmar]

Slow idle return says vacume leak to me. Generally in those situations the ECU has difficulty compensating for it causing the revs to hunt around until it stabilizes at the correct idle speed. So if the idle speed fluctuates up/down when returning to idle, or if sudden load is introduced from the cooling fan or AC there is either an air leak or dirty throttle body/ISV valve.

First thing to check is leaking hoses around the intake manifold, a garden hose and trickle of water will find them fairly quickly, if any water gets sucked in you will hear the engine stumble and idle speed drop a little, or use carb cleaner and the idle will raise a little. I have seen a few cases of leaking injector o-rings causing this on 5cyl motors.

If no leaks are found clean the throttle body and make sure the edges of the butterfly are shiny and clean, build up of dirt/carbon can hold it open slightly and let too much air passed when closed. And as above clean out the ISV and make sure it can move smoothly. Also worth checking the throttle cable, if it is adjusted too tight it can slow down the throttle body closing as it has to tension the cable spring when closed. To check hold the throttle body fully closed with one hand, then pull the cable outer with the other hand until the cable slack is taken up, then place the metal clip on the next position out from the one thats lined up. You want a little bit of slack so it dosent go tight as the engine moves around under load.

Also pay attention to what your boost gauge is telling you, specifically the negative pressure side. Does it hold a strong vacuum at idle, does it sit steady at idle or does the needle fluctuate back and forth. And how does it respond when you rev the engine up and close the throttle. You should be seeing at least half a bar negative pressure or ~ -40cmhg at idle, the more the better.

[Boghopper]

Ok, got round to adjusting damper, and sorted.

Just backed it off until it JUST touches before the throttle stop.

Its a different vehicle now!!

Thanks for everyone's input


Pete

[Tonybolony]

Evening All,
I've only had my s2 for a few weeks, and not driven another one before, so I don't know whether this is a 'characteristic' or a fault? - Even whilst driving spiritedly, when changing up thro the 'box , on hitting the Loud pedal again- there is best part of a second delay until power comes back on again. Is this normal for the ABY, or is there another function of the aforementioned Damper to hold the butterfly open just long enough to keep the boost on? Or am I completely barking up the wrong tree?

[Boghopper]

Hi Tonybolony,

What you are describing is not the same problem I had. My revs were staying up when I changed gear, making the gear change rough. I still had/have the same symptoms as you.

When you come off the throttle to change gear, you lose the boost pressure, and when you get back on the throttle it takes a moment for it to build back up. What you describe is perfectly normal.

Rally cars have anti lag systems to reduce this effect, however its not suitable for a road car and not good for the turbo.

Our cars, being 15 years old ish, suffer from this more than a modern turbo'ed car.


Cheers

Pete

[sinc1964]

Idle Control/Stabilization Valve , remove and clean it out with a good solvent such as parrafin etc.

If you need to replace it, only fit a genuine Audi part. Ebay is full of cheap rubbish replacements which do not work - I found out the hard way.

[Tonybolony]

Hi Pete,
I didn't mean to give the impression that I thought it was the same issue, and as I said- I wasn't sure whether or not the lag was peculiar to mine, or a general characteristic.
Anyway, the question I was looking to get an answer to is- could this damper be adjusted to the point where revs could be held up (momentarily) to keep the Rooster-Booster spinning during gear changes, in much the same way (But maybe not so brutally) that your average 17yr old apprentice plumber would do in his works van.
Feasible??

Tony.

[prj]

Hi Pete,
I didn't mean to give the impression that I thought it was the same issue, and as I said- I wasn't sure whether or not the lag was peculiar to mine, or a general characteristic.
Anyway, the question I was looking to get an answer to is- could this damper be adjusted to the point where revs could be held up (momentarily) to keep the Rooster-Booster spinning during gear changes, in much the same way (But maybe not so brutally) that your average 17yr old apprentice plumber would do in his works van.
Feasible??

Tony.

No,

And from your description you need to replace your dump valve, as that is probably not operating correctly.

[Tonybolony]

Cheers PRJ,
Thought it was worth asking.