I only have a pdf of the driver's handbook and no other docs. Low coolant light comes on - I checked the reservoir and it's down about half-filled. I didn't see any markings. Is this right or is the reservoir supposed to be close to full when completely cold?

 

Bottom of the fill neck is the full mark. It may go down to about 1-3 mm lower and stabilize there.

You either have a coolant leak, or your coolant reservoir cap is not maintaining pressure anymore. According to your description, the level is low and the coolant level sensor is working as it should.

Get the system pressure tested. It should hold 15 psi for at least five minutes. If it doesn’t hold pressure, you have a leak, and it should be easy to spot now. If it does hold pressure, buy a new cap.

 

Coolant reservoir tank level sensors can fail.
There is a float and magnetic switch correlation for the level sensing.

Sometimes the switch assy falls out of the bottom of the reservoir. (check the sensor switch)
Sometimes the float sticks to the bottom.
Sometimes things just 'wear-out'.

 

I get that and it was my first assumption from researching and knowing what to expect. But after a week with the car I actually looked and I found the reservoir about half-filled and wasn't sure what the level is supposed to be. Sounds like the reservoir tank should be almost up to the top. I'll add and see if maybe I have an easy fix.

 

As stated above, correct level is to the base of the filler neck:




The XK8 doesn't normally lose coolant. If the level has dropped significantly, the cause should be investigated as this engine does not take kindly to overheating.

Graham