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The bypass valve is known as a BPV, the boost pressure control valve is known as the BPC.
You can inspect the BPV for mechanical condition. Remove the return hose. Loosen the vac line. Engine at idle, apply and remove vacuum. Use finger to feel for valve opening and closeing. Do avoid placing finger tip where is might get pinched. You can apply suction with a hose with mouth pressures and determine if the diaphragm is leaking. If diaphragm does not leak, and valve does not work, then the vac line could be spit or cracked somewhere. This line tends to rot out badly. The orifice at the TB could be blocked too. In anycase, with the vac line off of the BPV and engine at idle, you should be able to feel the presence of a vacuum with your finger tip.
The BPV will be open at idle is it is good.
If the BPV is good, then the BCP or turbo is bad. You can confirm that everything other that the BCP is good by unconnecting the line from the BPC to the wastegate. Now in a calm traffic situation drive with a gentle foot and see how much pressure you can get. The turbo will take you to 15 psi then the manifold absolute pressure sensor's signal will cause the ECU to stop triggering the fuel injectors. This is a fuel cut. It feels very alarming, but is quite harmless. Do this check in sencond gear. Things will be too abrupt in 1st gear. The turbo indicator should be going to around 40 or 50% thru the red on the boost indicator. If you get high boost then everything is normal. If you cannot get boost, then there is probably a massive leak in the turbo ducting someplace. And such a leak can be the BPV.
However, if the wategate acutator is not keeping the wastegate closed, this could lead to low or nil boost as well. With a cold engine, grap the wastegate actuator shaft and try to move it. It takes a surprising amount of force to overcome the spring force in the actuator. So it shoudl be hard to move. The actuator move out when enough pressure is applied.
To confirm that the wastegate actuator is working, you could connect the hose from the turbo compressor hose barb directly to the wastegate. When you do this you should see boost of about 50% thru the yellow. A non-functioning wastegate actuator is extremely rare, so this is probably not an issue.
A MBC will provide more power and the boost pressure will come on much faster. The 1st two gears will get 'full' boost. You don't have to adust the MBC to get the maximun boost. If you did that, you would have to make other changes to keep in safe operation.
Bad plugs can limit boost as well.
posted by 207.43.195...
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