The "invention" of the "closed crankcase ventilation" (CCV) system assumes that any pressure generated in the crankcase is more than offset by the vacuum drawn into the intake manifold thru the crankcase vent tube. In most vehicles, is this vacuum is regulated by the PCV valve. In the XJ, it's regulated by a controlled orifice in the valve cover, matched with a small diameter tube between in the intake manifold and the valve cover port.
This works fine when the system is new. When it gets old, and carboned up, it sucks less crankcase vapors. Add to this the fact that old and poorly maintained engines have more blowby, and you wind up with a pressurized crankcase, with vapors looking for any way out. They find an easy route back through the air filter.
Cleaning the CCV vent in the valve cover, and the orifice tube, will help this. Upgrading to the later style CCV vent tube is also helpful on early vehicles.