My light comes on, I add fluid and 100-150 miles it is down to 1" in the reservoir. Have a separate temp monitor and the engine stays below 206---always. Drove her over 1,000 miles with the light on and the temp was on the money. No heating problems.
Question, it there a way to move the sensor so it will stay off unless the reservoir is completely empty. .
2009 XF 4.2 supercharged with 65,000 miles.
Thanks in advance.

 

The initial drop is just the water getting circulated into the overcomplicated snake pit of hoses. The level sensor is bad so there is no way to have it say if the tank is actually empty. You can bridge the sensor lead with a jumper and the stupid warning will clear after a few drive cycles. Pretty sure on the 4.2 there is a separate temperature warning if overheating, but at this point I don't even care. The tank will need to be replaced at some point, it comes with the new sensor. My replacement tank is riding in the trunk waiting for the next oil change to swap it in. Old one lasted 5 years.

 

Thanks much for the reply. I Have added water/coolant 4 times to bring the water level up to normal. Lasts about 100 miles and back down to the 1" mark and the light comes on. I figured is was overflowing in the 100+ degree Texas weather. I can't find any leaks and the car runs at 206 with tank full of at the 1" mark. ????

 

The coolant is going somewhere, you shouldn't be losing that much every 100 odd miles. The tank shouldn't overflow as it's a closed system, and if it were you should be able to see a load of manky coolant around the tank.

It sounds like you have a leak, hopefully outside the engine and not the head gasket. The coolant hoses and seals seem to last about 10-12 years before getting a bit stiff, this can cause small leaks anywhere. If the leak is small the coolant tends to boil off quickly before getting too far and it can be hard to spot, especially if it's somewhere like the EGR valve that is tucked right at the back of the engine bay.
Favourite things to check are

• Water pump
• Radiators
• Thermostat pipe work that connects to the engine block
• Supercharger pump

you could test the coolant system using a pressure tester, do it on a cold engine and you might be able to see where the coolant is going.

 

Thanks so much five. All this sound great and am having it checked Friday. My only wonder is how I can drive 1088 miles with the reservoir at 1" and the light on the entire time and temp never got over 106. If I had a leak, one would figure it would not stop leaking at 1/2 gallon.
I do appreciate your thoughts.

 

That is interesting and I'm not entirely sure. My best guess is either that the water level has reached the same height as the leak and therefore can't escape anymore, or that extra air in the expansion tank is giving it more air to compress so there is less build up of pressure in the coolant system and its only leaking when it's under pressure.

I had a similar experience last year, coolant level kept dropping, turned out it was the water pump but it only leaked when pressure built up in the coolant system. When it was at 12psi you could see it seep around the seal, but at lower pressure it seemed fine.

 

I think I’m having the same issue. I have 5.0 SC v8 and it’s like 103-105 degrees past weeks and I keep filling up once car is cooled after parking overnight. I wonder if it’s cause it’s just so hot and our motors run very hot.

 

Another thought is that most materials expand when they get hot so a small gap could leak when its cold and then expand to fill the gap and stop leaking when it is hot. That could explain why you didn't lose a bunch of coolant on your 1,000 mile trip, yet keep adding for shorter daily commute trips.

 

Either way these cars shouldn't leak a drop of coolant. If they do, you have a ticking time bomb. I would not drive until you fix it. If the small leak turns into a big leak you won't know there is a problem until it's too late. Alu block engines warp easily and you could do massive damage in a minute or so.

Pressure test, find the leak, have it fixed. Just topping it up frequently is asking for trouble

Just my 2c. But read around the forum if you don't believe me!!