As well as my Omega's I also run a 1999, S Type (3.0 V6 SE automatic). It's currently on 95K miles and I have had it for the last 8 years.
I have been very impressed with it and apart from some out of service issues it has been reasonable to service and run (except the petrol costs).
There are a few well known problem areas. On mine i have done work on:
The battery lives in the boot (it's a big one and expensive) and they don't appear to last that long. Even though the car will start if the battery is on it's way out or below capacity you can get a host of dash warning messages predicting doom and gloom with just about every system in the car!! Don't panic, a new battery clears everything. Budget for a new one evey 2-3 years especially if the car doeas not get used every day.
There are individual coils attached to each spark plug. Due to poor water sealing around the windscsreen/scuttle area water gets channelled onto the rear coils and they fail. Individually, they are not very expensive (£35 ish) but if you have to change all six or eight then ouch.
The windscreen wiper assembly seized up due to rust and lack of use!! Simply DIY job once the assembly is out of the car or a complete assembly replacement from the jaguar dealership. Reasonably cheap.
The radiator reservoir spung a leak and was replaced (very common problem), About £100
The suspension bushes at the front have a hard life and have been replaced 3 times to-date. These use to be extremely expensive but prices from dealears have fallen quite a bit over the past few years.
Front wheel bearing failure. Replaced mine twice to-date. Bugger to get bearing assembly out due to corrosion.
Rear suspension bushes also fail. Use to have to buy the complete assemblies which were very, very expensive but now you can just buy the bushes.
Some people have reported gearbox problems (especially on the facelift ZF boxes). Extremely expensive and can make the car an economic write-off.
Rear exhaust hangers rust thru leaving rear exhaust boxes unsupported. Easy and cheap fix if caught early.
Problems with the aircon. Some pipes fail due to rust where the pipe is attached to the body.
Problems with heater system. A valve assembly hidden in the engine bay can stick/fail. Very common problem but a reasonably easy fix.
Check for rust behind the plastic sill covers. Some horror stories reported there but mine have been ok. The rust proofing is not that good on these early cars. When I bought mine I spent quite some time adding additional rustproofing. Well worth it.
Generally, service items have been very reasonable and it is quite DIY friendly. I think both egines (2.5/3.0 V6 and 4.0 V8)use a timing chain rather than a timing belt for its engine system so no snapped belts to destroy the engine. Unfortunately, there is no Haynes manual but you can get hold of Jaguar Technical Service manuals and bulletins via the internet.
I would also try the Jaguar dealership first for parts as you can be pleasantly supprised at the low prices (Ford legacy) on service parts and there is a good after market suppliers which can also help to keep the costs down.
There are also a couple of good S type forums which offer plenty of good advice and "Jaguar" magazines which often have useful articles on the S type and gives you addresses of parts suppliers.
Hope this helps.
