2000 XK8 Convertible, about 120,000 miles.

I've had the warning intermittently, about 6 months ago I replaced the Expansion Tank (upper tank) and the problem seemed to be getting better. A couple weeks ago I got "Restricted Performance" with a "Anti-knock" sensor code. That made it easy to find a crack in the rigid plastic pipe that exits the upper expansion tank and replace it with a flexible hose. The fluid was dripping onto the Anti-knock sensor connector. No more problem with the "Anti-knock" but I'm still getting the Coolant Low warning. The temperature gauge has never moved from "normal", and since the replacement of the cracked plastic pipe I haven't seen any significant fluid loss in the Expansion Tank.

There was a small decrease in fluid level in the expansion tank when I checked it cold, but from what I've read on the forum the float should be at the top of its travel still. What I've observed is:

Shortly after start up (engine cold) I get the Low Coolant" warning and it lasts for approximately 20-60 seconds.

If it's shut down and restarted I sometimes get the warning and sometimes do not. It doesn't seem to be tied to how long the car has been off.

Sometimes I'll get the warning after 10 minutes or so of driving. When I get it then it seems to last longer; like 2-3 minutes.

Based on what I've read on the other threads, fluid is ported into the Atmospheric Recovery Bottle at high pressures and then when pressure drops the fluid returns to the system via vacuum pressure. So I'm thinking that perhaps there is another hole or crack in either the Atmospheric Recovery Bottle cap or the hose going to/from the Bottle. This crack or hole would cause a weakened vacuum and thus delay the return of fluid to the Expansion Tank, causing the level to fall and a "Low Coolant Fluid" warning.

What say ye, oh lords of English Engineering?

Thanks in advance.

 

Probably the float/sending unit acting up in the tank. I would just replace the whole tank assembly. I did it recently and it took less than an hour and $50.

My DIY write up on it is here

 

Great write up, but I've already replaced that tank.

 

Even so, if it is happening when still cold the fluid has not yet expanded so the slow return is not likely the cause. Also the expansion happens because the tank is full and under pressure. I can't see a scenario where the tank can void itself of fluid. Is it possible that air in the system could do that?

 

My crossover pipe failed and I was getting the coolant message after I replaced it. Nothing worked to clear the message. Turns out, and I discovered this by accident, there was air in the system even though I filled everything to what I thought was capacity. After I shut the car off and waited an hour, I loosened the expansion tank cap to see if it was cool enough to check the level. The car "burped" a huge air pocket and some coolant escaped so I immediately tightened the cap again. I promptly got distracted and forgot about it until on my way to work the next day when I realized the warning was gone, and it has never returned.

 

Another thing you may want to try. If I run the pre mixed 50/50 Dex, I get the light. The float in my expansion tank must be super sensitive. If I get the device used to measure the coolant concentration, basically like a turkey baster, and suck out the 50/50 mix and add straight Dex the light goes away after one or two drain and fills on the expansion tank.
I've measured the 50/50 mix straight out of the container and it was oddly enough just below where the mix is supposed to be. There are videos on yt that show the bobber (?) in the tank won't float in water. Once I get the mix just right the light goes away and stays gone. I hate that light, it's unnerving. I've also read to burp, get someone to push the accelerator to about 2500 rpms with the cap off and when you see flow top it off and screw the cap on. This has worked for me.

 

is the water pump orig? They can bleed off real slow if they go. I would after having driven it and it has got cold rub your fingers UNDER the pump see if it is wet?

 

Thanks everyone, some good stuff. I decided to order a new expansion tank, at $45 it was a no-brainer. I'll let you know if it solves the problem.

 

Well, big shout out to Mr (?) Rothwell! The tank was at my door when I got home today. The tank came with a probe installed so I decided to keep it in case it had something to do with the probe and not the tank. The old tank came out pretty well, had to figure out the clip for the electrical connection, and the hoses were hard to wiggle off. But all seemed to go well. Fired her up and drove around the neighborhood for about 15 minutes. No light (huzzah!), and I didn't see any parts on the floor as I came back to the garage, so that's good too. Thanks for all the help everyone. And a meaningful New Year for all who celebrate.

 

After having my local shop replace the expansion tank cap on my 98 XK8, assuming it was the cause of a coolant leak, I suddenly have a Low Engine Coolant Red Light warning. Even after topping off, no change. So I turned to this thread.

I was about to use the directions for fixing the float. After draining the tank, I could not see anything that I wanted to deal with, so I checked to see how it could be removed and be replaced with a new tank, cap, and sensor unit.

While feeling around the bottom of tank for any fittings, I saw that the sensor was actually a little loose, and leaking coolant. So I just snapped it back into place. No more leaks and no red lights.

This may be a temporary fix, so I will buy the new tank, cap, and sensor and install it at my leisure.

Just another possible thing to check before a major repair bill.