Almost all xj overheating is due to one or both of two sources. #1 bad thermostat. #2 (and most likely,) the radiator is just plain clogged. A stock, clean, fully functional cooling system is more than adequate to cool a stock xj in essentially all situations. Sure, a sloppy, wore out viscous fan coupler will not help the situation but as long as the auxiliary fan comes on like it should it will still cool. For reasons that have never been scientifically explained, (although many have theories,) xjs just eventually turn coolant into mud and clog up the tiny tubes in the radiator. You can try flushing it with all sorts of fancy chemicals but your best bet is a new cheapo radiator from the zone. The expensive ones do not fair any better in my experience.
