As you’ve got the 5.0 litre and if you’ve never had to replace a coolant pipe, you’re doing well. I’ve had them fail
after 3 years. Parts aren’t that expensive and some can be done with basic tools. Consider it the cost of ownership after 10+ years.

Could be rear crossover, front crossover, water pump, thermostat, oil cooler pipe, or others.

You definitely have a leak somewhere. I recently replaced my rear crossover, thermostat, water pump and upper radiator hose just after buying the car (only as a precaution because of recommendations from other owners on this forum) and headed out on a 4,000 mile trip, only to find that the reservoir was losing 4 to 6 ounces of coolant per driving day (150-350 miles). I bought a gallon of coolant along the way and filled the reservoir every morning; never got a warning. Could not locate the leak until after the trip when I "borrowed" a pressure tester from my local parts store and found that it was the brand new hose that was leaking slightly in a banded seam. The leak was on the backside of the new hose and could not be seen, and nothing was dripping under the car. I put in a warranty claim on the hose and was waiting for a replacement to arrive when this one let go altogether and I had to have the car towed back to my house. Luckily, I was monitoring the water temp on the freeway via cell phone app and an OBDII reader, and pulled over to the side of the road when the temperature jumped up and set off an alarm, just as my low coolant warning lit up. Normal operating temp for both my 2010 XK and my 2005 XK8 is between 194 degrees F and 205 F (90 C - 96 C); I set my app to warn me at 215 F (101.7 C) as I have seen both of my Jags only go up that high when climbing long hills between Las Vegas and Los Angeles in the summer, (well below 245 F (118 degrees C), which would probably be catastrophic). Engine temp only reached 217 F before I shut the engine off but it rose so fast I could have destroyed it in just a couple of more minutes. It's running fine now with no issues, thanks for wondering...

Borrow (buy and return) a pressure checker from a local auto parts store (my local AutoZone takes a deposit by selling you the kit/tool then refunds 100% of your money when you bring the kit or tools back), fill the coolant tank, and pressure up to see if you can locate the leak. Alternatively, fill the coolant reservoir tank up to the max mark, start the car, drive around just enough to heat things up, then stop, open the hood, and see/listen if the leak is revealing itself. Be sure to carry some extra coolant with you and don't drive the car very far until you resolve this; it goes without saying that the cost of an engine replacement is much, much more than the value of the whole car these days...

Coolant leak
 

Yep, it’s a leak. Rear of the engine, if you lean over from the passenger side (US car) and look between the rear of the engine and the firewall you can see some fluid. Also some on the top of the transmission.
I assume these will be the expensive hoses to replace...

Yep, there it is...

https://cimg9.ibsrv.net/gimg/www.jag...a053c65d73.png

one of two likely locations are the heater hoses at the firewall where the ends are hidden by insulation. A bit of a PIA but not terrible labor cost. The other more common one on that side is the valley hose. That requires removal of the SC. No way around it. That's about an 11 hour DIY if I recall and a lot of guys that do it, also freshen up the SC while it's off.

I don't recall if you can get a bore scope under the SC at the rear to confirm.

I have a 2007 XK. I noticed a small puddle under the car. Pink puddle.
I traced the leak to the hose between the reservoir and a pipe below it.
is that a special hose? Or just a certain size hose?

For my '07 4.2 NA, the Service Manual's spec is WSS M97B44-D. See below.

Even Ford didn't have a specific matching fluid anymore, though I see redline's comment above that Orange Dexcool also meets the spec.

So after a long search, I found one the that specifically lists this spec.

It's Peak OEM Orange for North American Vehicles.

The spec is not on the bottle, but is in the spec sheet: https://asset.productmarketingcloud....9-b668fbb36660

On page 2, top. About $20 or so. May not be on the shelf at your local parts store.

Since it matches the spec (and for what it's worth) the color, I've been using it.

Again, the 4.2 naturally aspirated original Ford spec. Other engines, other specs, perhaps.

Panthera


https://cimg1.ibsrv.net/gimg/www.jag...d38883e416.png
https://cimg5.ibsrv.net/gimg/www.jag...33733554ef.jpg

Nothing special, just a coolant hose. Be sure to check that nipple on the reservoir, as that plastic ages and is known to crack (I keep a spare, as it’s likely to happen some day).

On the 4.2 NA, there is a s-shaped hose from the top of the reservoir into the engine.

There is a butt connector between two parts of the hose. It's some crappy plastic/compressed fiber thing that crumbles after a number of years and the hose sections separate. I replaced the connector on mine with a brass fitting and hose clamps. Works fine.

Water top off
 

I had the same warning light. Coolant level was just a little low but sensor was triggered. I topped it off with distilled water and problem solved.

Red coolant light
 

I had that happen. The first time it happened I added a small amt of water, maybe a cup. Next time I drove it the rad light came on again. Looked in filler and it was down about a cup. I didn't add anything and the next few times I drove it the red light came on again, but the coolant temp on the gauge didn't change and on observation, it was still down about a cup. My wrench friend says there is a sensor like a gas tank sensor that goes up and down and sticks, Ergo it must be replaced but isn't an urgent issue

Likely the rear heater crossover pipe, could be valley hose to. You’ll have to remove the supercharger, so best to go ahead and replaced ALL hoses and pipes so you only have to do this once and not pay for the same labour and related parts (gaskets) multiple times at least for the next 5-10 years. Check the supercharger isolater
slack too.

dexcool or motorcraft yellow

motorcraft orange (same as dexcool) is being discontinued

The Prestone GM DexCool one I listed earlier in the thread does meet that spec...so should be all good with that one ( and it's also Orange in color ).
https://cimg0.ibsrv.net/gimg/www.jag...5cbb2a4a7e.jpg

Ends up it’s the coolant manifold, which is a part I’ve not heard of... and the quote to fix is over 2,000 bucks. Damn.
Anything else I should replace down there?

they probably mean the valley pipe

Likely rear crossover manifold as I said previously. Part is cheap, just tons of labour. And everything else I said before. ALL pipes, hoses and supercharger isolator. Parts are cheap, and you don’t want to have to pay another $1500 or so for labor to do the job all over again for another failed part. Been there, when tech swore I didn’t need to….I needed, again.

I actually think it's this. (Which may also be the valley pipe...?)
https://www.partsgeek.com/gbproducts...QaAgabEALw_wcB

Just what I told them, based on your advice. I'll add supercharger isolater. (Although I did have the supercharger rebuilt at about 4 yrs, 40K miles)

Well check it for slack. Mine was perfect at 100k miles so not
all fail.