Here is some advice from the JaguarForums.com, where more tips and fixes are available.
Seems to be one of the most commonly discussed. Depends a lot on where your car is located in the world.
North American vehicles seem to blow out air-shocks much more than those cars in UK/EU, don’t as why cause it makes no sense. Replacement air-shocks are available from Arnott, either re-built or their own version.
Air compressors will wear out. Mine went at 90,000 miles. They will wear out eventually, but will fail much faster if you have a leaking shock and the compressor is running all the time to compensate for the leak.
The air suspension also loses it’s calibration on the front over time, because there is only one sensor on the left front, the right front will go a bit high after a while. The re-calibration is fairly simple with the right tools.
The wishbone bushes on our cars should be considered a consumable item. They will wear out every 30,000-50,000 miles.
Heater Core and Pump
The heater core in our cars blocks up eventually which means less heat getting to the drivers side (regardless of LHD or RHD) if you’re lucky it can be flushed out. If not then it’s dashboard out to replace the core.
The electric Auxilliary Heater pump will eventually need replacing, around 100,000 miles. It can also start to leak externally, so you can loose a little coolant.
Some have been faultless, some have been problematic. Anything from ‘The Lurch’ to the ‘Barking’ at downshift. Also ‘surging’ due to the torque converter clutch. Most have now been resolved by oil/filter changes & re-flashing the software. Several X350’s have needed replacement transmissions because of solenoids failing in the TCM. Generally the transmissions are good, but you only hear about things that go wrong on a forum
The transmission is sealed for life so there is no fluid change scheduled into the official service procedure. If you notice the ‘The Lurch’, ‘Barking’ or the ‘Surge’ then it might be worth getting the fluid/filter changed, and the TCM re-flashed. Others will say that as a precaution you should have the fluid/filter changed every 50,000miles.
Throttle Position Sensor
The TPS has a limited lifespan, could be anywhere from 50,000 miles to 150,000 miles. It depends a lot on the environment, hot climates will reduce the life of the part. Replacements are not available in the aftermarket, BUT the sensor is shared with a lot of other cars, so you can pickup a throttle body from an X-Type or S-Type 2.5/3.0 for a fraction of the price and just swap the sensor.
Corrosion and Rust
It’s an aluminium bodied car, so no issues with rust or corrosion right? Ah, no…
The alloy body can get filiform corrosion in some places, typically the inside lip of the front wheel arches, the bottom corners of the doors, and around the bottom of the rear window. There are several reasons for this, but fortunately it does not spread like conventional rust, and is reasonably easy to repair for specialists in alloy panels
Rust on the other hand can be an issue for cars in cold/snowy climates where the roads are salted. The front and rear sub frames of the X350 are steel and will rust over time when exposed to salted roads. Also anything made of steel under the car will be affected (bolts, nuts, suspension parts, etc.) It’s worth getting under the car & having a look at the steel parts if you are in the UK, Northern Europe, Northern states of North America or Canada i.e. where the roads are salted in winter.
The thermostat’s in our cars do not last forever. Consider a preventative swap-out after eight years or 100,000 miles.
Driveshaft / Propshaft center bearing and couplings
After 120,000 miles or so the center bearing in the propshaft tends to get a little loose, giving a slight thud when you get on & off the throttle. The center bearing cannot be replaced, only the entire propshaft.
The rubber couplings at each end of the propshaft should also be changed preventatively after 10 years, as the rubber will perish with age. These are not available for the X350, but the S-Type ones are identical.
Transmission Cooler Lines
These also have a habit of wearing out & leaking. Should be checked after 100,000 miles at every service
The OEM wheels made by BBS are not the strongest, and can often be bent by potholes. This is not an issue unique to Jaguar, but seems to come up fairly regularly.
Supercharged Specific Issues
The early XJR and SuperV8 had Brembo brakes. The OEM rotors are poor & warp regularly, giving a shudder under braking. Aftermarket replacement rotors of a performance nature are much better & do not warp like the OEM ones.
Diesel Specific Issues
The EGR valves will eventually block/jam up with crap. Expect these to need replacing at 100,000 miles.
Diesel Particulate Filter
The DPF should not need any servicing, but sometimes they get so full that they are totally blocked & need replacing, especially problematic for city cars which don’t get a good motorway run to clean out the DPF.
It also happens that the pipes running off the DPF can develop leaks, which can cause a lot of “unrelated” issues. So always check these pipes at every service.
Engine oil level
Keep a constant eye on the oil level, over time it starts to rise as diesel gets diluted into the sump. When you do an oil change don’t fill the sump all the way, just a little over the low point on the dipstick. If the sump gets full then the service message on the dash comes on, and it can happen within a few thousand miles.
Flex-joints before the cat converters
Are known to start leaking & you get diesel fumes in the cabin. These flex joints are welded to the pipes and are not available as a spare part from Jaguar, however are available in the aftermarket or can be sorted by an exhaust specialist.