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Discussion Starter · #1 ·
I've been attempting to fix the idle on my 1990 Audi 90 quattro, so far I've isolated the issue to the IAC valve. Car idles perfectly with Idle control screw adjustment, with iac unplugged, but when it's plugged back in, it won't idle. Anyone know how I would go about recalibrating or adjusting the iac computer? I'm currently looking through my Bentley manual, but it seems like I might need the version specified for electronic issues.
 

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Have you cleaned the IAC? Inspected the hoses for it, and other vac lines? My experience with this issue is normally a leak in the system VS needing anything recalibrated.
 

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Discussion Starter · #3 ·
Yeah sprayed it out with carburator cleaner, got a nice amount of carbon deposits out, but that didn't seem to do anything. It could be a vacuum leak, but since the car idled when it was unplugged, I just assumed it wasn't that, although you seem to know a lot more about these cars than I do. Any common lines that tend to break? I didn't hear a noise that sounded like a vacuum leak, although like I said, that might be wrong.
 

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It doesn't take much of a leak to cause a problem idling. Is this a 10v or 20v car?
 

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1990 Coupe Quattro (3B, K26/K27, VEMS), 2007 S8
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early or late 20V? The late 20v ISV's are partially open when unplugged (or if wire breaks) and then are closed with like 10-25% duty cycle (not exact numbers), and after that allow for more air as duty cycle increases. This type of ISV when unplugged will allow a fixed amount of metered air through, and reduce the impact of a vacuum leak.

I'd say if things change with the plugging and unplugging of the ISV, then the ISV is probably working and doing its thing. A jammed or nonfunctional ISV won't have any change occur when it's unplugged.

So many places for vacuum leaks on these cars with rubber that is now 32 years old, but that's what is sounds like to me. Good luck
 

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Discussion Starter · #9 ·
early or late 20V? The late 20v ISV's are partially open when unplugged (or if wire breaks) and then are closed with like 10-25% duty cycle (not exact numbers), and after that allow for more air as duty cycle increases. This type of ISV when unplugged will allow a fixed amount of metered air through, and reduce the impact of a vacuum leak.

I'd say if things change with the plugging and unplugging of the ISV, then the ISV is probably working and doing its thing. A jammed or nonfunctional ISV won't have any change occur when it's unplugged.

So many places for vacuum leaks on these cars with rubber that is now 32 years old, but that's what is sounds like to me. Good luck
Okay, thanks.
 
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