Gang,

Still trying to diagnose the 'pssst' noise my 740T makes under boost. So far all lines have checked out, including the lines in the dash. My mechanic friend is a little stumped. Thinking perhaps the CBV diaphragm. My car is prone to light backfiring at throttle lift, which may be another clue to this. Can any turbo experts out there let me know if I should be able to blow through the CBV diaphram? I can. If the diaphragm was ripped could it make or cause this noise? This car has a Garrett T25 with the CBV built on to the turbo housing. Also I want to try a vacuum check valve in the line between the frame trap 'tee' and the intake manifold. The FAQ indicates that some turbos have a vacuum check valve in the CBV/BOV line. I would like to know what model years this item is used on so that I can try to pick one up at a yard or the dealer to try in the manifold line. Any other comments or experiences on this are appreciated. Reading the FAQ it makes perfect sense to stop the ability to overpressure the crank case and blow oil out seals. I found this line was near or partially plugged while doing some routine maintenance after I replaced my intake manifold gasket. Replacing this made a world of difference to my idle, as I had a large leak at #1 cylinder. But I digress...!

Next step is to open up the diaphragm housing to take a look.

Thanks for all comments.

Sean
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91 740T M46