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The middle outlet (marked "W") is connected to the
wastegate, the top ("C") goes to the compressor housing of the turbo,
and the bottom ("R") to the intake. The boost is adjusted by
alternatively supplying the wastegate actuator with the pressure from
the turbo or venting it to the intake, thus allowing the wastegate to
stay closed at pressure levels exceeding the base boost.
The boost pressure control valve (BPC), also known as the APC
solenoid valve, is used on most Saab engines to control the turbo
charging pressure. The valves on Trionic5 models are often prone to
failure. In some cases it just needs a good cleaning, but sometimes it
becomes permanently stuck in one position, causing the car to either stay
on base boost, or go into overboost until it hits the fuel cut. This
article provides an insight on what could be casing these permanent faliures.