There is no accurate mileage prediction for when you will need to replace the shocks on your Jaguar XJS. The manner in which you drive and how much you drive your XJS will determine how soon you need to replace the shock absorbers. If you have noticed that the way the car handles around curves has become different (spongy) or that your car is beginning to bottom out when going over bumps or potholes, it is time to change the shocks. No matter what year Jaguar XJS you have, the process is almost exactly identical. You can change the shocks in about one hour.
Park your Jaguar XJS on a level surface. Use a lug wrench to loosen the lug nuts on one tire--it does not matter which. Jack that side of the car up till the tire leaves the ground. Replace the jack with a jack stand to support the car and remove the tire completely.
Remove the nut holding the bottom of the shock to the chassis with a socket wrench.
Push the bottom of the shock off the bolt coming from the chassis (some XJS models use a bolt that is separate from the chassis, pull this bolt out). Let the shock expand to its full length (about five minutes) before continuing.
Locate the nut holding the top of the shock in place. On front shocks it will be easily visible--the shock will be attached to a bolt coming from the subframe and into the wheel well. Remove the nut and pull the shock out of the car. On rear wheels, the shock will either pass through the body and subframe of the car and be accessible through the trunk or, it will have a hook design that catches the subframe but does not pass into the trunk. For the first, remove the nut and draw the shock out through the wheel well. For the second type, remove the protective bonnet from the shock (a plastic cowl that snaps in and out of place by hand), push the shock up into the body of the car until you feel the hook come free, turn the shock 1/4 turn and draw the shock from the car through the wheel well.
Remove the shock from its box. Do not cut the plastic strapping holding the shock closed.
Slide the bottom of the shock onto its attaching bolt behind the brake rotor and loosely tighten the nut. This will be the same process for both front and rear brakes.
Cut the plastic retaining wrap with a pair of scissors. Guide the shock, as it expands, onto the top retaining bolt (for the front shocks) or through the subframe of the car into the correct position (for the rear). If your rear shocks are hook-mounted in the subframe, you will have to turn the top half of the shock while it is expanding and hook it, by feel, in the subframe.
Tighten all the nuts completely and place the tire back on.
If you don't properly attach the top of the shock the first time, remove the shock completely, press down on the top half to compress it to its closed position, hold the shock closed and try again.
Do not attempt to puncture the old shocks to release the gases inside for disposal--the shock could explode. Always take your shock to a mechanic for proper disposal; there may be a small fee.