All 850's have an oil trap that is positioned under the intake manifold that is involved with ventilating the crankcase of blow-by. Though I have been fortunate not to have had problems with any of mine, I understand that it too can cause a back up of pressure. And it is not an easy repair job. The intake manifold needs to be removed to gain access.

Drawing blow-by out of an engine requires suction. The flame trap is located just prior to the throttle body. As air is drawn into the throttle body, it also provides the suction to the flame trap.

Since turbo's work with forced air to the throttle body, they can't use a flame trap but they still draw blow-by out of the engine and reburn it. There is a small hose that is attached to the air supply line right before the turbo where there is still suction.

Good luck.
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Don 93 & 95 854 GLT, 95 855T