About every 6 weeks I get the low coolant light, always after a spirited driving session(swift 0-60 sprints etc.). I top off the reservoir tank which is usually an inch or 2 down. I've had pressure tests done to no avail & can't spot any stains/smells anywhere under the car or around the usual suspects in the engine bay.

Any clues would be greatly appreciated.

 

The only other thing I can think about right now is your secondary tank, the one in the left fender.

If that one is leaking or low on level somehow, then there is not enough coolant there to be sucked back after your spirit driving --> higher temperature --> higher pressure --> coolant pressed out process.

I guess it is a long shot, but I can't think about anything else than the most obvious, that is your are loosing and/or burning it somehow.

 

Any links to the diagram of said tank would be greatly appreciated.

 

I don't think that exist.

It is located in the left forward part of the engine bay, but under the main beam, so all you see is the entrance cap.
Follow the coolant line from the expansion tank which goes along the left fender, and you will end up at the secondary tank.

I think you will have to take the front bumper off to check its condition, but perhaps you can do some investigate work without doing so.

EDIT: how old is your expansion tank cap?

 

I just received a new one today actually, I will put that on & see what happens.

 

Might solve the problem altogether, as it regulates the coolant to be sucked back into the main system after expansion.

Also know that the coolant level normally stabilizes a couple of mm under the neck, even if you fill it to the top.
It will be pushed put in the next heat cycle, and return to said level.

This is totally normal, and should still be far above the 'low level' setting.

 

Hi King Charles,

Hopefully the new cap will solve your issue, but if it doesn't, here's the parts diagram showing the expansion tank (part 1) and the header tank (part 5) that Eric mentioned:



The diagram is from jaguarclassicparts.com, the parts service of the Jaguar Daimler Heritage Trust.

Cheers,

Don

 

Kudos sir !

Well I guess I have to thoroughly test the new cap theory. To make up for my rare spirited drives by going on an extended romp, condensing my normal 6 week ration into a day or two lol (sinister laughter bellows).

 

Well even after the new cap it's back, right on schedule lol. The saga continues,,,

 

I am not sure I mentioned this before, but there is perhaps one theory left for your problem (if it is not a system leak).

If the secondary tank (item 5 in above diagram) has a small leak, it might be that the level there drops such, that there is not sufficient fluid left to be sucked back after you spirited ride.

I can imagine your engine gets hotter if you really push it, the coolant expands more, and there is more of it temporarily pushed into this secondary tank, failing to return into the system while cooling down.

Just some free thinking...

 

That seems very plausible in my thinking, the game is afoot lol !

 

What ever you do, DO NOT put a stop leak additive.

 

No worries there, I know better lol.

 

A sticky float? There's been a number of posts on this warning over the years, particularly in summer months. Limescale coats the rod the level float slides up and down on, it sometimes catches on a low point and you get the warning.
Fixing this is a simple job I call the finger trick - open the header tank once cool and locate the float with your index finger and slide it up and down until it moves nice and free. Job done.

 

Good tip ,but that doesn't account for the missing coolant each time.

 

Another one is a hairline crack in the header tank, only when hot enough, then coolant leaks out generally around the join. You'd have to remove it to inspect it fully. XK's have this fairly common one.

 

Mine does EXACTLY the same. ALWAYS after some sprinted driving (higher revs than normal). I don't normally use any water. All I am sure about is that my atmospheric pressure bottle (as it is called) is rather filthy inside and that I wouldn't be surprised, if the fluid in there is limited in it's movements back in to the normal water bottle (i.e. something blocking the return flow etc.).

 

During my knock sensor repair it revealed I had a small weep/leak in the usual suspect area/the thermostat tower. Replaced w/ the Welsh aluminum kit & all's well.

 

Did you checked for any fluid stains/puddle under the car when parked ?

Not sure this can help but..

As I systematically check the coolant level before running the car (cold) I noticed some months ago there was some liquid missing in the coolant tank !.. no more than an inch driving the Cat for a 500kms ride or so ..

I found the leak coming from a heater hose's non return valve situated close to the firewall in the engine bay !

The leak was coming from an invisible hairline crack on this non return valve !

Once the part removed and cleaned it was impossible to notice the crack on it !









The heater system have 2 non-return valve !


When I bought my XJ in 2013 , I also had to replace the thermostat tower (plastic part made) by a new one (metal) as it was starting leaking!