On my way home from a 2000 mile trip, I got the "Low Coolant" warning message. When I did get home, the expansion tank was dry. So I put in about a half gallon (or two liters) or maybe some more. I ran the engine until it was fully warmed up, but could not see any moisture anywhere. No drips either. I have not taken off the numerous plastic engine shields (yet) so my examination is so far a cursory one. The fittings at the expansion tank seem sound, at least the two on top.

What is the best diagnostic strategy?

 

Pressure test would be best, and quickest, but may need to take out under tray to see if anything is dripping on that.

 

Is it a supercharged version ? There is a coolant hose in the valley of the V engine that is hidden by the supercharger. If it leaks the coolant exits at the rear of the engine. A 2000 mile trip would be enough to lose that amount of coolant as it's under pressure. Could also be the header tank where the small diameter hose goes in. The attaching nipple cracks so its only a very small leak path, so the coolant leaks out but soon dries leaving no liquid. You should see staining though.

What puzzles me is that you had lost so much coolant before the alarm came on. On my previous X350 only about a cup was lost before I got the alarm. Did you carry on driving any distance with the low coolant alarm on ? Very unwise if you did.

 

My XJ8 is not supercharged. The low coolant light came on about 20 miles from home. The temp gauge was still in the middle, so I was not too alarmed. More interestingly, the low coolant light went out, and then on, and then out again during the 20 mile stretch.
I did not see any obvious signs of leakage around the overflow tank. After I refilled the coolant, I idled the car for a half hour or so, and closely watched the tank. Did not see any moisture, so I am still puzzled.
So my plan at the moment is to drive carefully, watch the tank level, and look around the engine for obvious leaks. If it gets a LOT worse, then the problem will be at least identified.

 

The most common leak location is the header tank, mainly where the small diameter hose fits on. The attachment nipple gradually cracks all round until the hose drops off, but well before this, the low coolant alarm will alert an owner. I had this go on my previous X350, and also a water rail connection at the other end of this small hose.

Look all around the engine in the dark with a flashlight. At night beads of coolant will glint back at you, but whilst they do this in daytime too, your eye is adapted to daylight so you can't see them.

Other location might be the water pump seal.

 

Can the water pump be seen without taking off plastic covers?