Audi 5000 idle problems

[John Sherrow]

Tom,

Below is the text from an old post of mine regarding a similar idle
problem.

 The car had a rock solid 800 rpm idle. Suddenly while driving idle
would not decrease below
 1200 rpm. Would climb to 1400 rpm then drop again to 1200. Frequency
was about one cycle
 per second.

 Airflow passes thru the MAF (metered air) and enters the intake
manifold at the point where
 the "Michelin Man" hose connects to the intercooler. If you remove the
MM hose you will see
 the throttle plate and a cast air channel which allows air to bypass
the closed (at idle)
 throttle plate and enter the other end of the intake manifold via the
ISV. This is why the MM
 hose has the tear drop shape.

 With the throttle plate closed (foot off the gas) the throttle switch
directs the ISV controller
 (marked as Idle Volume Control on my fuse panel, gotta love the
translation) to regulate air
 flow into the intake manifold using the ISV. The ISV contains an air
regulator disk controlled
 by a solenoid. With current applied to the solenoid the disk will move
toward the electrical
 connector providing a path for air to enter the intake manifold. With
no power applied a
 return spring returns the disk to the shut position.

 It was suggested that a vacuum leak could cause the problem. It was not
clear to me how a
 leak could cause the engine rpm to increase. It seemed that the
increase in rpm was likely
 due to an increase in metered air and the engine computer was
responding accordingly. I felt
 a vacuum leak would cause unmetered air to enter the intake manifold
resulting in an idle
 stumble.

 As suggested I cleaned the ISV and then tested it with a 12 volt/half
amp power supply. The
 disk moved freely and snapped shut when power was removed. One notable
difference was
 that the replacement ISV would not pass air when I blew into the inlet
side of the ISV. The
 problem ISV would allow air to pass.

 I believe the return spring in the failed ISV was fatigued from years
of fighting the solenoid
 and no longer has the spring force to seal against engine vacuum thus
resulting in the
 increase in "metered" air during idle.