The oil comes from the PCV system, which injects oily crankcase vapours into the air inlet, downstream of the MAF, but before the turbo. The turbo itself gets hot enough to burn off carbon deposits (unless used for a lot of short runs where it never gets up to full temp), but the EGR system injects hot, sometimes sooty, exhaust gas into the oily inlet charge. In bad cases the EGR pipe gets enough oily sooty crust to restrict the airflow.
Some folks fit catch cans in the PCV system to reduce the oil content, or sometimes vent the catch can to air and plug the air inlet connection.
Some folks fit catch cans in the PCV system to reduce the oil content, or sometimes vent the catch can to air and plug the air inlet connection.
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