This trick comes from Chris 9-5, a moderator on the SaabCentral forums. It was in a thread from July of 2007. I found it while searching for Forge diverter valve information. The OP had posted about weak turbo pressure and Chris 9-5 suggested the following modification:
Remove the top vacuum tube from the boost control valve and then plug the loose tube.
Chris 9-5 explains that it bypasses a one way valve that can become faulty.
It worked wonders for the OP. Chris 9-5 recommended that only people with weak turbo symptoms should make this modification.
As far as I knew I didn’t have any problems, the car drove as spirited as the day I purchased it. I had all the power I needed, the car was quick… so I naturally decided it wouldn’t hurt anything to try this trick.
The results were dramatic! It is like driving a different car. My whole driving experience is totally enhanced. The most noticeable change is when the transmission kicks down at wide open throttle on the highway. I really feel the torque steer for the first time, just like the OP in the thread described.
Here is how I plugged the vacuum line; I found a bolt that was the right diameter and cut it down to about an inch long. I then painted the head to make it look nicer. It’s that easy.
Some people in the thread didn’t see a difference when they removed their line. Other people tried it and found the change remarkable. The people who have made this modification haven’t seen any problems and I will make a post if I see any negative results on my 9-5. I’d say people should give this a try and see if they get back some turbo pressure. Just revert it back if there wasn’t a positive change in performance.