Crankcase filter oil return pressurized

[Rawze]

First of all, any buildup of crank case fumes on an ISX engine will quickly wipe out the overhead cams, because any pressure buildup will cause restricted oil flow to the top end and starve the cams of oil. In fact that is one of the biggest reasons that the ISX has cam issues.. lack of proper engine maintenance and neglect of regular servicing of the crank case filter.

That engine should never be allowed to have any kind of crank case pressure buildup ... meaning that the crank case filter needs regular replacements and not be neglected.. or better yet.. to install a maintenence-free crank case filter for it and be done with it...
ref: http://rawze.com/forums/showthread.php?t...50#pid8950

If the crank case relief circuit cannot keep up with the blo-by, and it is not clogged up, etc ... then your engine is likely in need of an inframe. - How much blo-by does the engine make?.

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As far as oil getting into the air system,... have you checked to see if the cac piping going into the intake manifold is not wet with oil on its insides??. Checked to see if the turbo is not seeping oil out into the intake?