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Re: ISV problems

From: Meron

>>If the connector is unplugged, then the spring
>>should close the valve, dropping the idle speed well below normal (as
>>controlled by the physical idle screw).
>Thanks for your comment,
>I was under the impression that the ISV is designed to allow the system to
>idle during a failure (closes by the spring).

That's right.  The ISV controls the amount of air that bypasses the 
throttle.  There is also a mechanical bypass (a brass screw in the throttle 

>When my ISV failed, it closed
>for a while (wondering idle between 900 to 1200 RPM) but shortly opened and
>started to surge, I plugged the engine openings and the car went back to
>"wondering idle" 900 to 1200 RPM.

When ISVs fail, they usually don't fail completely, but become sticky.  If 
the engine revs are detected as being a bit low, the controller increases 
the drive to the ISV.  Due to the increased friction, the ISV doesn't budge, 
so the revs drop a bit more.  The controller increases the drive still more. 
 All of a sudden, the ISV opens as the driving force overcomes the friction. 
 The revs shoot up.  The controller senses this and lowers the drive.  The 
ISV sticks.  The revs rise still higher.  The controller turns off the drive 
completely.  The ISV unsticks itself and closes completely.  The cycle 
continues.  This is only one source of failure.  Squirting carb cleaner 
through the ISV (or better still soaking it) sometimes helps things.

If you disconnect the ISV (or if it fails in the closed position), then your 
idle speed will be dependant on the brass bypass screw.  This usually gives 
an idle speed of about 700/800rpm when the engine is warm and <500 when the 
engine is cold.  This often causes the engine to cut out, particularly when 
you load it by turning the steering wheel (if you have power steering).

On my 80q, the controller failed.  It had a blown output transistor.  This 
had gone short circuit, so my idle went up to 3000rpm.  When I replaced the 
transistor, everything was OK for a few days, and then it blew again.  Being 
too mean to buy a new controller, in the end I built a prototype circuit to 
regulate the idle speed and this has worked perfectly for 3 years now.

1984 Audi 80 quattro
1989 Audi 100 Avant