One more thing about the suction-back of the coolant from the overflow tank to the header tank: both o-rings on the cap need to be in good condition and sit tight in the filler neck, especially the larger o-ring that sits approx. 5 mm below the top of the neck; also, the internal surfaces of the filler neck need to be smooth with no scratches or irregularities.

If there is a small leak pass the larger o-ring, the coolant, while being pushed out, may not leak pass it and onto the top of the header tank as the coolant, once pushed pass the pressure valve in the cap, is not at any pressure, i.e. it flows freely down into the overflow tank. However, during the engine cooling down, a fair bit of vacuum is needed to pull the coolant up and, if there is some leak pass the larger o-ring, ambient air will be pulled-in instead.

 

Next week when I recheck the coolant level, I will inspect the O rings on the cap to see if they are tight & undamaged & feel the filler neck to see if it is smooth.

Since I only check coolant level when cold (After car has sat all night) Any coolant in the fender overflow tank s/b already back in the expansion tank, assuming everything working as designed.

If coolant level in the expansion tank (Header tank) is once again a couple inches below the bottom of the filler neck, I will not top off. I will wait & see if the low coolant light comes on.
If, in fact, there is insufficient vacuum for coolant to be sucked back into the expansion tank from the overflow tank, as designed, then I assume my overflow tank may have overfilled with all my topping off.
So, where can this excess overflow tank coolant go? If there is no vacuum to allow it to go back into the header tank?

Is there a simple way to look at the overflow tank to see if it is full?

 

I suggest yo check the coolant level every day, if driving the car. You now have a new(?) tank and level sensor installed and you do not know if the sensor is actually working (unless you have already tested it). This is to avoid, if the sensor is not working, the coolant level going too low.

If, for some reason, the coolant is not sucked back into the header tank and the overflow tank gets full, the coolant will be pushed out of it and on the floor. The overflow pipe entry into the overflow tank is not sealed and the excess coolant will come out there. You can see the overflow pipe entry point from the engine compartment, just follow the plastic pipe #4 from the header tank (see pic below). See if there are any traces of coolant around the pipe entry point into the tank #5.

 

I took a pix of my ‘new’ black cap.

I have not driven the car much (10mi) since added 12 oz on the 12th. Coolant level ~ ˝ - 1” below neck. I did not/will not top off anymore. I used flashlight & followed small hose (#4) to try to see overflow tank. I could not see it. Anyway I did not see any evidence of a leak where the hose disappeared, which may be the top of the tank & just doesn't look like a tank to me.
I checked the cap inside & seems like O-rings are tight. Large O-ring feels smooth. I felt all around inside neck with my finger & very smooth.

I will keep my eye on the temperature gauge & check coolant level day after any drive over ~ 10 miles. I do not think the coolant level drops at all when car just sits.

Usually 5-6 days/week the car is moved from my driveway to the street & back again at night.





my 'new' XJ8 coolant cap

 

My coolant level is also about ˝ - 1” below the neck and remains there, even though I filled it (when recently replacing the coolant) close to the bottom of the neck. The genuine cap has writing on it, also showing the part number so you cap and tank are probably aftermarket but that's ok. One thing you can easily do is test the level sensor - switch the ignition on, push gently the float down with a thin wooden stick (or something non-magnetic) and have someone check whether the warning light is on. If it works correctly, then you don't need to check the level so often but rely on the warning light.