Well… now you’ve inspired me to raise the hood… in my housecoat. (I’d wish you stop doing that;-)

 

Back.

Ok, there is no air movement in or out of my reservoir cap. And it has never leaked.
We need to get on top of this. Yes, the deformed threads are disconcerting. At the parts counter confirm the new cap goes onto, and can be removed from a new reservoir without issue…

 

@guy Well, you're dedication's most appreciated, housecoat and all!

So..... sitting overnight, expansion tank empty again. Topped off to proper fill level. Had thoroughly washed the two caps and dried overnight. The new cap refitted and did not appear to leak air with the hose squeeze test so ran the engine for a few minutes. When it got up to temp, coolant started rising and upper hose would not firm up. Turned off the engine and checked the cap, it was hissing again

I tried the previous cap, which's more like two years old, and it was so loose I didn't feel comfortable bothering to try it. I've got a brand new aftermarket cap, never used because it feels cheap and weighs less than genuine caps by quite a bit and the plastic's much thinner than genuine. But since I had it, went ahead and gave it try. Ran engine and soon upper hose firmed up and ECT A stabilised at 199F, ECT B at around 117-122F and coolant level remained unchanged. Went for a long drive with a few brief stops and all operated correctly. Had a 90 minute stop, then returned home. A bit worried as ECT A this time wanted to be at 222-226F and ECT B at 90-96. When I arrived home I checked the tank and coolant level was just above max fill like yours and upper hose was very firm but NOT rock hard.

I've crossed everything I've got for good luck, couldn't be this easy could it? Did I really end up with a defective cap from JLR?

 

Have now done hundreds of miles. Most of the time ECT A ranges from 190-203F. A few times I've seen it spike to 236-244F cold in the morning. Coolant level does not ever go down and mostly stays at the same level, occasionally going just above the max fill line. Upper radiator hose softens up after an hour or so and softer overnight but never completely soft. Still using the cheap knock off cap. Exchanged the defective one at the main dealer but hesitant to fit as all appears well.

Crazy I went through all of this for a brand new defective cap BUT it does make perfect sense as all the symptoms match a cap which either would spontaneously not seal, releasing the pressure and causing coolant to boil over or not releasing pressure, overpressurising the circuit.

Hope I've not jinxed myself here...

 

I’m relieved for you that this is sorted. Chock it up to a lesson learned. I would never have guessed a faulty cap would have the cooling system this way.

Your pain, my gain.

Cheers!

 

All's well so far. ECT A still mostly staying in the 190s. I've never watched the temps this closely so really no accurate idea of what they've ever been but I did watch the ECTs on the Jag a few days ago and they behaved quite similarly so I reckon temps can fluctuate greatly.

The symptoms were all there, the challenge I think was not accepting 2 failed genuine caps back to back even with one being brand new. But there you have it.

Many thanks for your assistance @guy

Cheers