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In gears 3-5, you should have boost that goes to the red line and stays there, perhaps with a small overshoot at first.
Check out some basic stuff. The pressurized turbo plumbing may have leaks. There are 4 host clamps, starting with the compressor discharge, and going to the intercooler. There are 6 hose clamps to check from the intercooler to the throttle body, including two at the bypass valve.
With engine idling, remove the bypass valve discharge hose. You can now feel, perhaps see the valve plug through the discharge port. When you remove the vacuum line from the bypass valve, you should feel the valve close, and open when you re attach it. Don't get your finger tip pinched when you do this. Next inspect the end of the vacuum line for splits, and also at where it fits onto a hose barb at the front of the throttle body.
Next, remove the bypass valve, note it's orientation first. Clean it up, and apply pressure by mouth to the 1" fittings to see that it is not leaking. Apply suction to the vacuumm line fitting to see that there is no leak there either.
Check or replace your spark plugs, use heat range #6. Clean your injectors and combustion chamber with one or two treatments of Chevron's Techron Concentrate. These should help temperature or Air/Fuel ratio related knocking, which would reduce pressure via the APC function of the Trionic ECU. A dirty air fitler could also elevate temperatures by forcing the turbo compressor to be working harder thab neccesary.
All of this is inexpensive. The hose clamps at the intercooler can be a pain, or at least a few bloody knuckles :) Pick some Techron, and let it work, then you can get at the other stuff next weekend. Use the highest octane pump gas that you can get, and perhaps change brands.
If after doing these basics, if the problem persists, then you can contemplate the BCP valve and other possibilities.
How many miles? engine oil?, normal octane used? K&N air filter? How long has this diminished boost been going on? Any recent dealer or mod work that could have led to this?
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