The price of caps does vary a lot. Years ago on RockAuto.com I saw that I could purchase the whole expansion tank (with cap) at just a few $ more than the cap alone, I opted to get the tank. That was a few years ago, so prices may have diverged since then.

I'm still waiting to use that new tank, the cap that came with it is still in use and OK after 4+ years.

Z

 

I don't care what the cap says on it, as long as it functions as needed. I am considering buying a second as a backup. (You can't be too careful these days.)

 

My radiator cracked and decided to replace cap along with radiator due to possibility cap was not opening to expansion tank. I know not to tighten too tight because the o-ring makes the seal. One from RockAuto broke first time tried to remove. Top with threads separeated from portion that goes into reservoir. RockAuto sent another and it again broke. RockAuto refunded and said wouldn't send another because kept breaking. I think they were Gates brand. Went to a salvage yard and found an original.

 

I just got a call from Consolidated Auto Works, and the problem wasn't the cap; it was an improper clamp (on the expansion tank) that caused the coolant leak. The issue has been resolved without the need for any more replacement parts. The oxygen sensor and all the labor was $625.25 (USD)..

My cat is on the road again.

 

I call the XK8 "Ian" and on the road, he runs like a stallion! That 4.0 Liter V8 has enough "get-up-and-go" to get the job done. People still admire the old boy...warts and all.

 

I'm glad it's sorted out. I'll share my experience. One symptom of this problem is steam coming out from under the hood (the coolant drips onto the exhaust manifold), and it smells like hot coolant. Be careful with the clamps if they're threaded; in that area, they should be flexible. If the clamp is too tight, the heat can deform the expansion tank inlet. The hose has probably expanded.
This happened to me. In the end, I put black silicone sealant on the lower tube of the expansion tank (of course, with no water in the cooling system up to that level, and never put this silicone on the hose). Then I attached the hose with two flexible clamps and waited 24 hours before adding coolant. I know that when I take it apart again, I'll have to clean the inside of the hose very carefully (or replace it, but that area is tricky). No coolant has leaked since. Besides that, as I mentioned, the connector wasn't making good contact.

 

This, among other positive aspects, is why Jaguar used the constant tension type hose clamps. They are superior to the ubiquitous worm gear hose clamps.

Z

 

"The coolant drips onto the exhaust manifold." - jhuertas
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”….Be careful with the clamps if they're threaded; in that area, they should be flexible. If the clamp is too tight, the heat can deform the expansion tank inlet…..”
This, among other positive aspects, is why Jaguar used the constant tension type hose clamps. They are superior to the ubiquitous worm gear hose clamps. - zray.

This is precisely what happened. Baron (the mechanic at Consolidated Auto Works), discovered someone had placed a ring clamp on the hose connected to the expansion tank, which caused the aforementioned leak onto the exhaust manifold.
When Baron replaced the upstream "B" bank oxygen sensor, he discovered the offending clamp and replaced it with a genuine Jaguar clamp. This resolved the problem, and relieved the attendant gastrointestinal distress. My poor insides feel MUCH better!