Can drive the 1988 XJS for 10 miles or so, then the temp guage will hover rig before the light turns green and the needle will sit under the "N" which i lov Sometimes the same trip will result in not running hot at all........any ideas ??

 

My ideas,

Air in the syetem, and maybe the overflow tank is full of MUD, coz they do that.

Fan hub is getting lazy.

Radiator is dirty on the INSIDE and OUTSIDE.

Thermostats are having a female moment.

Please check out the XJ-S section, as there is an active thread at the moment regarding cooling systems. Mainly to do with Electric Water Pumps, but I have a few answers in there that may help you a tad.

 

Quin,

I've moved your question from General Tech Help to XJS forum. Members here with the same model will be able to help.

I recall the XJS V12 cooling system runs to its own set of rules!

Graham