My experience with Jag coolant leaks began 4 years ago when I bought a 2017 XE 3.0 replaced the cross over pipes myself as it was 2% beyond warranty. A new leak has recently started. Since it's my daughter's car, I assumed new water pump would do the trick, but the leak is apparently slow and came back.
Currently the symptom is that over a day or so of light driving the valley will fill up with coolant and start sloshing out the front. What is strange is I can't figure out where it comes from. It's not the new water pump.

I've learned how and where to look with an endoscope. To start, I syphon out the valley with a 1/4 inch suction tube then look at particular spots with the scope to check for fluid puddles. After an hour drive no leaks or fluid is in the valley. But by the next morning about one tick on the reservoir is lost and 3oz fluid in the valley on the passenger side. The absence of fluid by the front cross over pipe and water pump to oil cooler pipe suggesting they aren't the source of this new leak.

I'll admit there are pink crystals on the back of the heater manifold and evidence of dripping, but away from the valley. It's possible those deposits are a result of splashing since no wetness has been seen yet. Has anyone had coolant in the valley that was due to the heater manifold? Currently I'm not convinced that the heater manifold is leaking coolant into the valley.

So all this seems to point to the oil cooler. The dealer quoted $750 for a new cooler but was truly shocked and said they are rarely changed. Some on this forum/YouTube have pointed to the oil cooler as a source of coolant leak, but not sure how they know. Is there a bench test to determine failure if I decide to pull it?

My next step is to apply some pressure to the reservoir cap - is there a specific cap I should buy and any precautions. Hopefully I can see the level rising in the valley and still no drips from the heater manifold, cross over pipe or water pump pipe. Perhaps I should just tear into it, but I gut would be to change the heater manifold and ignore the oil cooler, and I'm trying to avoid doing it twice, so and advice would be appreciated.

 

My shop always replaces the oil cooler gasket while the super charger is off. And I’ve seen kits that include new oil coolers. I asked my dealership if they ever change the oil cooler during the crossover pipe service and they said no. I was asking because I was suspicious that my shop may be inflating the work.

If you see signs of leaking at the manifold pipe I would definitely change that out as its failure rate is near 100%.