Two weeks ago I received a low engine coolant alert. Up until this time, I had not received any notifications or noticed any "burning" off while driving. The reservoir was quite low, so I filled it to the reco'ed level. Yesterday I took the car out for a quick trip (5 or 6 miles total). On the way home, while I was at the stop light, I noticed coolant steaming on US driver's side near where reservoir sits. When I popped hood, there was a small amount of coolant visible beneath reservoir (I took some pics). I checked reservoir, and it only had about an inch of coolant in it. Thought I'd reach out to all of you to get your insights?

Of possible note - most of coolant hoses were replaced about 2 years ago (less than 2K put on since replaced). A busted hose then, at that time the spraying was quite visible underneath the bonnet ... this time, while I did see some visible spray and spotting below the reservoir area, nothing to the extent of my previous coolant issue. Planning on having the vehicle looked into early this week, but wanted to reach out to the real experts first .

 

It's not unusual to have a reservoir leak.

 

Just did this job for the same reason.
The complete reservoir from Amazon was less the $25 and it came with new cap and sensor.
Get you a turkey baster when to remove as much as you can and keep the reservoir up in the air so not to loose any and be ready to pull the hose and replace the bottom one as fast as possible.
The biggest pain it the plug on the sensor, it's one of the wire ones, need to pull the wire up to get it off. Only one Torx screw holds it on, the two stabilizers at the back just pull out, take the top front and back hoses off after you get it empty.

 

I've already mentioned this in another thread. The same thing happened to me. After replacing the coolant reservoir, steam and the characteristic smell came out (the coolant dripped onto the exhaust manifold). It was the lower hose. To fix it, I applied black silicone sealant (the kind used for gaskets) to the tube that comes out from under the expansion tank (not inside the pipe), reconnected the hose with two flat spring clamps, and waited until the next day to add the coolant. No coolant has leaked again in over a year.
I removed the coolant to the level using a thin tube of the type used in pneumatics and a syringe (as many colleagues do), so you don't have to drain the circuit completely.
If I have to open it up again, I should remember to use the silicone sealant to prevent anything from getting inside the pipe.

 

Although I’m positive this repair falls under the heading of shade tree hack, I’d like to have you with me when I get stranded on an uncharted island.

I’m pretty sure you could make a radio transmitter from 2 coconuts and a shark tooth.

Z

 

Perfect, assuming there are coconut trees, the problem is getting the shark's tooth. We could do the rest between the two of us.

 

Try with a new cap.
When the coolant gets really hot, its supposed to lift a valve and displace coolant to the auxiliary expansion reservoir, when the coolant gets cold again, it suck back coolant from it, keeping engine circuit topped up at all times. The XK engines are made to run quite hot, hence the somewhat, in my opinion, overengineered coolant system. If seal fails, cap with its valve fails, "coolant level management" fails. Unless the reservoir is cracked of course.
I took the seal from an old oil filter, that flat about 4mm wide seal, cut it to a perfect fit and replaced the old squashed to oblivion seal in the cap. I made sure i hade the correct level in the aux reservoir (behind the RH front wheel), about half, topped up the coolant. -> Happy days.

/E

 

The coolant cap on the XK8 is set at 100 kPa, the one on my S-Type 3.0 at 120 kPa, and the one on the Citroën C2 (yellow) at 140 kPa. So, the water is warm, but it's not one of the hottest cars. Since pressure is relative, water at 2 bar boils at around 120°C, at 2.2 bar at just under 123°C, and at 2.4 bar at a little over 126°C (if Google's AI is correct, as I haven't checked the tables or done the calculations). At those temperatures, the caps should release pressure. In the case of the XK8, I think that since there isn't room for a larger reservoir (coolant + vapor or air), it must use an auxiliary tank, which also ensures the coolant level in the engine is maintained during expansions and contraction of the fluid. Please correct me if I'm wrong, because that's what I've thought since I discovered the presence of this atmospheric tank in the xk8. The proper functioning of the cap is essential.

 

Folks - Just following up, great feedback from all. The problem ended up being the reservoir. Replaced that and the cap... good as new. Thanks again for the great advice, much appreciated!