Crappy Jaguar coolant reservoirs
 

I bought a used 2005 x-type wagon and its many defects. I am considering replacing the coolant reservoir with a 2003 lincoln ls coolant reservoir. I figure it cost less and likely is better quality but would it work? I mean its the same engine so why wouldn't it?

Comparing the two designs (see Rock Auto for pictures) I don't see why you'd want to try this. Even if you could match up the hoses there's no way to secure it to the car, so it'd just bang around in the engine compartment. Yes, this part may be a weak point but it's just plastic - as on all cars. They fail over time because of the heat cycles. If it does fail and you (or another driver) don't recognize it quickly enough the engine overheats and now you're faced with a much bigger problem. It's worth replacing as you bring your car back up to snuff.

It appears that there would be fitment problems. The main fault with the oem recovery tank is the small nipple near the firewall. The spring clamp ears are mounted facing up and hit the hood putting pressure on the nipple, causing it to crack. The Dorman replacement is reinforced with a brass tube in that area and fits perfectly. Took me 15 minutes to swap out and have had no problems since.

I don't know why you would assume that the Lincoln part would be better quality. All modern cars use plastic reservoirs and they can all suffer the heat/age cracks that yours has.
Replace yours with a Jaguar OEM reservoir - perfect fit, no worries for another 10 years.

Stops leaking when hot
 

Thank you everyone for your input. The seller of the car told me he has to replace it every two years and the jaguar salvage yard owner said that was common. Maybe it's the California heat idk. But the seller also sold it to me woth green coolant and no oil on the dipstick so perhaps it was the result of a neglected cooling system. The entire car was neglected. I feel like I rescued it. No jaguar should be in the condition this one is in. But I plan to restore it as much as I can afford to. I plan to flush the system and use orange coolant and replace the rherm,hoses,reservoir,and water pump. I hope I wont need ro replace the radiator but if I do I will. Strangely it only leaks at start up and when cold. Once heated up and in hot weather the coolant stops leaking. What's that all about?

Could stop leaking when it warms up because maybe a hose or fitting expands enough to seal it up.

Is it leaking from the reservoir or just leaking from somewhere? If it is leaking from somewhere the water pump would be the first place I would look. They seem to have a shelf life of 90K-100K miles on these cars.

Good for you for giving a neglected X-Type some love to keep it on the road. Hope the neglect wasn't so bad it becomes overwhelming. Once you take care of the main issues hopefully you will have something pretty reliable that doesn't overwhelm your wallet.

If car is leaking coolant I recommend you find out exactly where the leak is before throwing parts at it. Put front end of car up and take down the splash shield so you can get a good view from below. I recently replaced a leaking radiator (my DIY is posted here); radiator is not that expensive ($150, Rock Auto) but front bumper has to come off to get everything loose to get the radiator out the bottom.

To fix that leak nipple near the fire wall cheap and easy: Cut the nipple at the base, buy a brass hose fitter (at Home Depot or Lowe) same size or slightly bigger the original nipple, the one with one thread end, drill the old hole at the nipple base slightly smaller than the threaded end of the brass fitter (smear some silicon sealant on the thread) and tread it into the drilled hole, I've fixed two of my previous X-Type this way, it's cheap and work nicely, cost only less than $3 for the brass fitter, why spend some $40 for a new tank and it will leak again sooner or latter