Fun with the V6 ISV (or "Honey, you said this would only take 30min")

[Chris Newbold <cnewbold@transarc.com>]

Over the weekend, I decided to clean the ISV on my 1993 90. I've been having
some problems with bucking transitioning on and off the throttle and, when
cold, the engine stumbles immediately after firing. So, with a bit over 70k
on the clock I thought it would be worth it to make sure the ISV wasn't
sticking...

The ISV appears to be readily accessible on the 12v V6: remove the plastic
cover around the intake manifold, and there it is, mounted vertically to the
left-rear corner (while standing in front) of the manifold, right next to
the throttle body. In reality, it's a little more complicated, as are all
things Audi.

It's mounted to a plate on the manifold by three studs, about 3" long. So,
you need considerable clearance to slide it off. Here's the procedure which
I developed empirically:

1. disconnect the 2-pin connector from the top of the ISV

2. remove the throttle cable adjuster clip to allow the cable to be moved
   around as needed

Almost directly below the ISV is a metal coolant pipe which comes out of
the head and runs laterally...

3. loosen the retaining nut for the A/C refrigerant hose bracket which is
   attached to this pipe; it it not necessary to actually remove the
   bracket; you need to do this so that you can move the hose to get the
   ISV out

4. roll back the rubber boot and disconnect the 3-pin temp. sender connector
   from the sensor which is mounted on this pipe; this connector otherwise
   prevents you from sliding the ISV off its mounting studs

5. remove the two 10mm bolts which attach the plastic intake plenum to the
   intake manifold; these are the obvious two, right on top; slide the
   plenum straight back about 3/4"; this is needed to get the accordion
   hose out of the way

6. unbolt the three 10mm nuts retaining the ISV; you'll need a ratchet and
   something like a 6" extension; be careful not to drop any of the nuts,
   they'll be lost forever; you'll have to wiggle the socket in there between
   all of the hoses (hence steps 1-5!)

7. slide the ISV off the studs and lift straight out

Reassembly is roughly the reverse...

The ISV appears to be different than those I've seen described here before.
This one is manufactured by VDO, and has two adjacent ports on the mounting
face on the side. The control mechanism is in a vertical, cylindrical casing
at the top. Air flows in through one port, turns 90deg. and goes through a
U shaped passage, turning 90deg. again on the way out the other port. Sort
of like the trap you find in a sink drain.

The working part of the ISV seems to be a "plunger" which comes down from
the control mechanism above. It's positioned like a stopper in the top
of the U shaped passage. The plunger itself has slots cut in its
circumference. A stack of thin, disc-shaped plates are visible through
these slots. At rest, the plunger appears to completely block the passage.
(Though air may be able to flow through the slots in the plunger?)

I did not have time to fool around with it to see whether the plunger
moves up and down or if it rotates when power is applied...

I cleaned it out with O2-sensor safe carb. cleaner; it wasn't very dirty
at all; just a slight coating of oil from the PCV system.

And, of course, the cleaning had no effect on the symptoms I've been seeing...

-Chris
1993 90CS 70k miles