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2.7 biturbo

The selenoid valvefor charge pressure control changes the opening time to
atmospheric pressure according to the signal it recieves from the engine
control unit(duty cycle)
Thus a control pressure is produce by modulating the charge pressure and
atmospherics pressure. This pressure acts on the pressure unit for the
If there no flow the valve is closed and the charge pressure acts directly
on the pressure unit. The wastegate opens even if charge pressure is low...
If charge pressure control fails the charge pressure is thus limited to a
"basic charge pressure" in order to prevent the maximun permissible charge
pressure being exceeded.
This result in loss of performance...
The "basic charge pressure" is the charge pressure(approx. 300 - 400 mbar)
which is achieved without regulation(mechanical charge pressure)
The wastegate is kept closed in a depressurised state by a spring inside the
pressure unit. The entire exhaust gas flow is routed via the turbine and
charge pressure built up.
The control pressure countreact this spring force and opend the wastegate.
Part of the exhaust gas flow is fed from the wastedgate past the turbine,
and the charge pressure stops rising....