One evening while driving I noticed that when I lifted off the throttle while under boost that the turbos made a lot of spool-down noise--a lot more than they had made earlier in the day or any time before.  This clearly was not normal for my car.  It does have a very small amount of spool down noise but this was a dramatic increase in noise.  At first I thought a hose had ruptured or blown off but the car ran perfectly fine under boost... it was just noisy when I lifted off the throttle.  The following morning I started digging around to track down the problem.  Checked all the hoses, everything was fine.  I hooked a hand vacuum pump to one of  the bypass valves and  gave it a squeeze.  Nothing.  I pumped it quickly a few more times and still nothing.  I could easily blow into the signal hose... mmmmm, ruptured diaphragm in the valve.  I checked the other valve... same thing.  Both of my valves were toast.   

I suspect that one of them probably failed some time ago and the remaining valve was serving double duty.  Once the second valve failed boost pressure in front of the throttle would build up after closing the throttle. This excess pressure was, in essence, loading (or stalling) the turbos so they made more noise as they slowed down against the higher pressure. In normal operation the valves are somewhat proactive about dumping boost as manifold vacuum opens the valves. The new stock valves started to pull open with just 2" of vacuum supplied by my hand pump. With ruptured diaphragms the valves become totally useless.

click to enlarge images

	Drivers side valve diaphragm .  Any wonder it wouldn't hold vacuum.  Diaphragm turned inside out for viewing
	Passenger side valve diaphragm.  Same general location and slightly larger.
	Twins at birth and twins in failure. Sucks doesn't it... or is that blows?
	The guts after autopsy

Closeup of my stock valves.

I had to replace the valves ASAP. What to do.  Not willing to part with over $300 at this point in time for aftermarket valves I decided to go with OEM bosch valves again and just keep a close eye (or ear) on them.

I purchased the valves for a Porsche 993 TT (Porsche # 993.110.337.50). These were reported to be a better valve and a direct replacement for the S4 part. When I got them home and inspected the valves I discovered that the Porsche part had the SAME Bosch number as the original valves on my S4 so back to the dealer to investigate.  The parts guy (excellent parts guy, IMO) looked up the valve for the 944 turbo. The listed part superceded to the Bosch 108 valve (the S4 valve). He continued digging and spent a considerable amount of time looking up other turbocharged Porsche valves. Everything we could find eventually superceded back to the Bosch 108 valve.  Ok, what about the OEM Audi Sport Quattro valve Audi #035 145 710 (Bosch 0 280 142 102). This valve was recommended by a few folks and mentioned in the AudiWorld performance forum as the way to go for OEM. The valve was also reported to be used on the 944 turbo.  No dice... leads right back to the Bosch 108 valve (again, the stock S4 valve).  What the....

Soooo, he made a call to Bosch to get a Porsche or Audi number that crossed to the Bosch 102 valve. He was informed that the 102 valve has been replaced by the 108 valve.  Oooook. End of story.  The Porsche parts were less expensive than the Audi part number (go figure huh?!) so I just used the 993TT number to save money.

Our research suggests (unless the Bosch guy gave us bad info) all that these part numbers represent the same part:

• Audi/VW# 034 145 710 A (stock S4 part)
• Porsche 993# 993.110.337.50 (plus other Porsches that all ended up at the Bosch 108 valve)
• OEM Audi Sport quattro Bosch#  035 145 710 (reported to be a Bosch 0 280 142 102)
• Bosch # 0 280 142 108
• Bosch # 0 280 142 102 (replaced by the 108)

If you know of accurate conflicting info on this issue please let me know... 

The original valves survived 16,000 miles.  At some point in the future I may go with better valves, I just can't part with over $300 for valves at the moment.

Update

I've since replaced the OEM valves with Forge DVs.  Info here.