So I recently purchased a 2004 XJ8 from a private party in Seattle. It has about 130,000 miles on it now and is running perfectly fine. The problem is that the car will throw a pretty consistent Low Coolant warning, which then goes away a few minutes later. It generally will not come back in a normal car trip of 30 minutes or so. I don't believe it's a random electrical glitch because it happens very consistently. Here's how it works. I start the car cold each morning to drive my kid to school across town. I drive on a few side streets and then get on I-84, which is about a 8 minute drive. At nearly the exactly same spot, it throws the warning, which is also almost exactly when the temperature reaches "normal". I then drive for about another 7 or 8 minutes and the light goes off right about the time I get to school. Total distance is about 10 miles or 20 minutes max.

I plugged in my Bluetooth OBD reader and OBD Fusion app and watched the engine temperature. It sits right at 90 degrees C and doesn't move. There is never an indication of overheating by the temperature gauge on the dash. From this I infer, that the coolant system is working as normal. There are no other P codes and no Check Engine Light.

I read through the various posts and came to the hypothesis that there is a problem with my coolant expansion tank. I believe that I can do that job as a DIY project. Before I do so, I wanted to test the theory of a sensor related problem. (I do not see any evidence of a leak anywhere.). I opened the coolant tank and extracted about a quart of fluid with my Mightyvac extractor. I clearly sucked out enough to get the coolant level below the lower "stair step" plastic indicator in the coolant reservoir. I thought I would see if the system would throw the Low Coolant when there was actually low coolant. To my surprise, after running for a few minutes, the coolant level was right back to where it was before I extracted the coolant. I understand that there is supposed to be about 10 quarts of fluid in the system. I did not get a Low Coolant warning. This confuses me, as I do not understand how the coolant could return to the same level. I am tempted to suck out more than 1 quart of coolant fluid to see if it keeps happening, but, on the other hand, I don't want to damage the car by running it with excessively low coolant.

Does anyone have a theory as to what causes the coolant sensor to come on erroneously yet systematically? What would you recommend at this point?

If I do move forward with the coolant tank replacement, I believe I can extract the fluid, get the tank out, and put in the same amount of fluid. However, this begs the question of how I know if I actually have the correct level of fluid? It seems like the system will fill up the coolant tank even if it's "low" by 1 quart. I don't want to overfill it and damage something.

Thanks!

 

Is it possible that the system was serviced and not properly bled before your purchase? The Jaguar coolant system is more complex than my 1970 era vehicles and it is a little more difficult to purge all the air from it's system. Perhaps a pocket of air is trapped in the system and as the coolant expands when the engine is warming up it pushes the coolant ahead as it circulates. You will not see a temp increase when the "bubble" gets to the sensor but it would show as a low condition. I would review all the information others have posted about doing a coolant change and drain out your system then flush and refill with new coolant. Be sure to use the correct antifreeze as the different types do not mix well. Since you know you removed some coolant and it is appearing to be full I would suspect air in the system.

Good luck.

 

+1 on the above.

The best way to refill and purge the system is with a vacuum device for that purpose however failing that:

When refilling have the nose of the car up on ramps or a slope at least.
Put the heat oh full. With engine running until no more coolant can be added.put cap bacl on reservoir.

Proceed from there.

Make sure you are getting good heat flow from the interior air vents.

 

The expansion tank sensor can be quite sensitive. I had one that would trip if the coolant was down less than half an inch in the unit. I later replaced the entire expansion and sensor and issue was gone. I haven't seen a low coolant in about a year.

You need to open the bleeder screw when filing - This is in line with the small hose off the back of the tank and has a large X for a screwdriver - don't use a phillips unless you have a giant one. a good large flat blade works best. After completing the beeding operation, if you still have issues there are numerous aftermarket expansion tanks - I wish I could find the one I bought, but its lost to the sands of time.

https://www.scribd.com/document/3606...-bleed-r-i-pdf

 

I made an effort today to bleed and top up the reservoir and it seems to have had some positive effect. I drove it around town and did not get the Coolant Low warnings. One theory is that the previous owner, who did a DIY job on the thermostat, did not bleed all the air out. The problem with this theory is that I drove the car 1,000 miles including 500 miles from Seattle to Boise before the warning showed up. This leads me to my current theory that the system has a small leak somewhere and that it actually was low. I believe I put in about 6 ounces net through my three bleed/fill/cool iterations today. Maybe that could be enough to get the coolant level in the right zone. I guess I will drive it for a while and see if the message comes back. If it does, I think I should UV dye test, then pressure test the system and see if I can find and repair the hopefully tiny leak. If there really is no leak, I will go the route of replacing the coolant tank and flush again on the theory that it is a finicky sensor.

One of the forum posts suggested squeezing the coolant hose near the expansion tank to "burp" the system. When I do this, it seems like I can get bubbles but they are not significant. Am I correct that these bubbles are not the same as a "real" air bubble that one hopes to work out with a bit of trial and error?

Thanks for the tips.

 

You are probably correct that you only see small bubbles caused by the rapid movement of coolant back and forth in the hose when squeezing it. On the long trip it is possible that the coolant was circulating fast enough that the sensor would/could not set a warning message signal. My experience has been that cooling system problems are not as easy to observe on longer trips when I am driving fast and steady as when I am in stop and go traffic even if the shorter trips do involve some faster driving. Also there may be other things that change such as running the heater or less. Sometimes even how I am driving may make a difference. Especially if I push it a little harder or just spend more time at higher rpm in lower gears. The posts relating replacing hoses, expansion tanks, water pumps, heater cores and such all share the same problems with purging the air from the system. That is one of the great things about the posts that share the resolution of the issues encountered. Since you have noticed some improvement you may well be on the right track. If the PO did replace the thermostat that will give you other things to keep in mind. If the thermostat was replaced did the housing also get replaced? Good luck.

 

It sounds to me like you have a faulty level sensor. If your coolant is at the correct level and you are getting level low warnings then I would say the float probably has a small amount of coolant in it. Air trapped in another part of the system wont give a low level warning also it makes no difference about what setting you have the heater on when bleeding as there is no valve in the system, the heat is controlled by blend flaps in the heater box and hot water flows through the heater rad all the time

 

11. SET the heating system to MAX heat, the blower motor to MAX speed and the air
distribution to instrument panel registers

The above is taken from the Jaguar workshop manual.

I have to presume Jaguar had good reason for those instructions.

 

To close out the thread, this issue was resolved by replacing the coolant expansion tank. I do not believe there are any leaks in the system, as it has been a week without the warning light.

The cost was $57 for the part (Meyle) + $77 for labor and a gallon of coolant from my local Boise independent.