Problems in my ownership;
Fuel pump failed.
Wheel bearing failure O/S/F
Heater blower stuck on high, resolved by replacing the Darlington transistor.
Before I got it;
Most suspension bushes had perished, and a rear shock was leaking.
Rusty floorpan and bulkhead/arches.
Aerial mast sticking.
Diff bearings noisy.
Some common things are/things to look out for;
Rusty bootlids / arches / bulkhead / sunroof / pan sides.
Headgasket's around 120k
Bushes on suspension (heavy car).
Some electrical issues, blocked water drains etc.
Appetite for rear shocks.
Door handles (early)
Dirty earth connections cause VCM warnings.
Caliper bolts missing (causes wheel to lock up when braking).
ABS sensors on Pre-90's (Girling brakes only)
Fans in engine bay
Water can enter behind the dash and at best wet your leg, but in the worst case will short out your Main ECU!!
Aerial masts if not cleaned
On the up!.
Engine/Transmissions are usually solid even after 200k miles.
Interiors wear little, only drivers side seat cracks and tears but this is expected.
Lots and LOTS of spares floating around, midlife crisis, many being broken up now, jaguar still has new original parts although there costly. something like 208k! 40's were made.
Lots of models to choose from. Some quite rare indeed. (Daimler Majestic LWB, XJ12 is also uncommon now).
Overall a very very nice car, previous "owner" unloaded lots of problems onto me. So I took the car to a Jaguar specialist and had it inspected; it wasn't good, with lots of issues.
Even though the car had lots of problems, it was still a good ride!
I had all four of the brake disks/pads changed, rear A-frame bushes and rear shocks changed, upfront I had new wishbone bushes/and shock bushes. After putting on 4 new tires (P6000), the car handled 1000 times better, and it's much much smoother.
Even though it's around 1.9 tonnes, it still does pull well at 3.5krpm +. Brakes have excellent fade resistance, and it makes a great engine note!. Transmissions are solid.
Interior is pretty amazing for a car of this age; I think its better than the new models, more space etc.
It's not massive on fuel; maybe get 22-24 MPG on a good run, and the massive tank holds around 86L's incl expansion space etc.
Even though it's cost me a lot to fix up these issues, I can say I don't regret buying it one bit! Looks so good in motion and when stood still.
If your looking to buy one and want the best,= chance of getting a reliable one, buy a 1992+, 1994 being the last year of XJ40 production. Early ones 87's 89's are actually a very different car; Ford helped Jaguar change and resolve a lot of issues. Later is better. Later cars 93-94 main changers are Airbags, battery in boot and on some they have a X300 hybrid suspension.
Hope this is useful in some ways. Take someone who is familiar with the cars when looking; problems can be easily spotted when knowing what to look for. I made a mistake not doing this! But no car is without its faults.