I've tried the soaking approach with no results. There are two things you can do: Bosch will rebuild these for a rather hefty price (about $300), and it will return with a two-year warranty. Check out the Bosch Automotive Tradition website: here
For the time being, you can bypass the AAV by plugging the top port on the valve and the corresponding port on the manifold. The car will run like utter crap when cold, but should be fine at operating temperature.
Did you ever get issue resolved? I had the same problem and it was the auxillary air slide valve...which are nearly impossible to find new since they have been discontinued. I haven't found anyone who remanufatures them but heard that someone is out there.
I recently got on Facebook and joined the 164 group. Great resource of information. Actually, my car has a page not me.
Ebay is a good source of parts amd you can get your rotors there...or anything else you need. I actually bought a car just to strip it down.
My number is (937) 361-0378 just in case you have any questions.
i finally found an aux air regulator from a guy in Minnesota named Volvo Resurrected...had to make a gasket for it but it seems to be working...had to get the A/C hose manufactured cause I could not find one anywhere...I pick up the car from the mechanic bruvetomorrow...hope everything works...I will check out the FB 164 page tonight...thanks for the update...and the info...