My 03 MCS just all of the sudden decided to stop making full boost. Was driving down the highway and got on the gas and the car was really sluggish. Pulled off and did a run to redline in 1st and 2nd and only saw a max of 4.5psi. It usually runs 12psi. There is no check engine light, there were no "bad" noises, belt slipping etc. Just a sudden dramatic drop in maximum boost. It seems I can also hear some whistling maybe under WOT that sounds very similar to the bypass valve. I did a quick look at the bypass valve and with the car @ idle it will pull hard open. Car off it is not hard up against the stop. Doesn't seem as springy as it should. Could be the bypass valve spring let go?

 

Could be the BPV. How old is it and have you ever changed it? If it is the original, you are long overdue for a new one.

 

Original valve with 54k on it.

 

Installed a Detroit Tuned valve over the weekend. The stock valve had definitely gone bad. Not sure exactly what broke but there were bits rattling around inside the diaphragm housing.

As far as the Detroit valve goes the biggest improvement I noticed over the (working) stock valve is throttle repsonse. Boost comes on much quicker. Especially in the lower gears the car reacts to throttle inputs much quicker. I also had some lag and hesitations before the DT valve which so far after a few days of driving seem to be completely gone. The Detroit Tuned website claims an increase of boost pressure of ~1psi. My max. boost pressure did not change at all. That's not why I bought it so no complaints on my part. Just though I should put it out there.

I should also note I was running the VGS both before and after the DT BPV install.