A good place to start on most any early XJ is by checking the air filter to see if it´s oil soaked. Cleaning the small plastic line running to the top of the valve cover (unhook it from the intake manifold and blow through it, if you can). Then cleaning the oil and baked on junk out of the throttle body and the idle air controller and the seat. Be carefull not to spray much of any kind of liguid on the throttle position sensor. Some people clean the TB on the vehicle, I´ve always been a bit shy of washing all the crap down into the cylinders and always remove the TB. Reusing the TB gasket isn´t advisable, but if you do, a light coat of grease and making sure it lines up exactly, usually gets a good seal. Be carefull of the plastic vacuum lines, they get old and brittle. The TV cable (to the tranny/AW4 auto) adjuster can be knocked out of wack, a little mark on the sliding adjuster, will tell you if it´s moved.