Arrived home after a small trip to some drips of coolant under my 85XJS. Lifted the bonnet and discovered a small weep from the partly displaced level sensor in the expansion tank. I gently pushed it back home into the rubber grommet and when I let it go it blasted into the engine bay courtesy of 16psi and the freshly lubricated connection between the pin and the grommet!!! Too hot to stem the flow digitally …so I opened the radiator cap and went hunting for a low collection vessel. Lost a lot of coolant!!! Pulled the expansion tank out. The sensor is a smooth pin of uniform diameter pushed into a rubber grommet. The friction was apparently going to hold this pin in position against 16psi coolant pressure forever…and if you believe that then I have some original steel off cuts of the Eiffel Tower for sale at a “special” price…!!!
Nothing leapt out at me as a permanent fix for this terrible detail so the tank is in the “naughty corner” until I have a fix for it.
Any fixes implemented ??
Yeah, I replaced mine with a stainless steel screw and nut, along with a spade terminal. You should know, though, that I was relocating the sensor from the radiator and hence was able to drill the hole for it exactly where I wanted it – which was where I could hold that nut in position with a long grabber while screwing the screw into it.
Hmmm. Disturbing story.I try to make my car as bullet-proof as possible, so this was worth investigating.
I replaced my header tank a few years ago, along with other coolant parts, but held on to the old one. I found the old tank, and down to the basement we went. It did not take much effort to remove the probe, and it is not even visible from the fill tube. I have not looked at the tank in the car, yet, but it seems to me that the probe could be prevented from escaping by a wrap of mechanic’s wire around the whole header tank. Insulated of course. I don’t think loctite or weatherstrip glue would be helpful.I’m glad my car does not have the earlier thermal fan switch (otter?) that was a push in fit also. More investigating following morning haircut. (and more coffee.)
Hmmmm…zip ties not really an appealing option…although would work…the rubber expansion nut is an interesting option…but would need some thought/surgery on the original sensor pin…
Toss the original pin. I’ve never seen a rubber expansion nut before, but it looks like it’d work great combined with a SS screw and a male spade connector. The only concern is making it watertight so coolant doesn’t leak through the threads. Use a regular screw with the male spade connector installed with a nut tightened up against the head of the screw, preferably with some Loctite to both secure and seal it. If the spade connector is brass or copper, it might even help seal the assembly. Then screw the whole thing into the rubber expansion nut. The screw should be long enough to extend about the same distance into the coolant that the original sensor did.
The early XJ-S had the sensor at the top of the right radiator header. My experience has been they work just fine for many years. It’s when they harden that they fail. The reason is the pressure differential inside to outside squeeze the rubber axially, and Poisson’s effect results in radial expansion, sealing tighter. That said, I would replace after removal, and never lubricate.
Ed,
I couldn’t agree with you more. I have these Coolant Probes and Bushings (C43221) in three of our driveable Jaguars (2 V12s and 1 I6) and all three of our XJ6 parts cars had them and I never had a problem with any of them. Nor do I ever recall seeing them mentioned as an issue on any of the Jag-Lovers lists. I think the discussion about alternative designs is a solution looking for a problem. 
If I ever had a coolant temperature probe pop out I would simply get a new bushing (less than $10) and install it and the probe without any lubricant. As you mentioned, it may have failed due to age, or possibly because it was lubricated when it was last replaced.
I know that some folks look for any reason to “upgrade” their cars at the drop of the hat, but I really don’t see this as an issue requiring anything but a new bushing. But hey, that is just me.
Paul
Interesting. 7 bucks from Amazon. 25 from a Jag supplier. My original lasted 30 years without leaking, so the one in the car now should outlast me.
Dave,
Yep. But if you just bought a new coolant level probe bushing for say $7 plus shipping you would miss out on all the fun double guessing the Jaguar design engineers, spending endless hours and dollars trying to come up with a solution that Rube Goldberg would be proud of, and then studying and measuring coolant levels forever to see it was working or leaking, or? Then there will be the fun when the next owner posts on Jag-Lovers asking WTF is wrong with my Low Coolant Level probe?
I think you can guess what my opinion is on this “fix”. 
Paul
I think I can guess! Before I retired I spent several decades being the “Go to” guy in a W.Pa pipe mill. Responsible for heavy equipment. If it wouldn’t start- Call The Limey If it had a flat, was stuck in a snow drift, Etc, Etc, Call the Limey.The machinery had to run- RIGHT NOW!
Consequently, my usual response to a problem is- How can I get this fixed PDQ! Buying the correct part wasn’t always an option. Maybe I miss the challenge. Nowadays, though, just FINDING the correct part can be the challenge!
Regards, Dave.
I’m not smart enough to figure it out, but I suspect there’s something in the design (so long that the rubber isn’t hardened) that causes the internal pressure (which is applied uniformly over the surface of the bushing) to fix the probe to more securely as pressure increases, sort of like Chinese handcuffs. Maybe not; just a suspicion.
Yep. Ed Sowell already mentioned Poisson’s Effect in an earlier post in this thread. This is what holds the bushing in place when the coolant level probe is inserted into it.
Paul
Yes, probably right, but I still can’t figure out exactly how.
With respect to the OP’s problem (ejection of the probe, not the bushing) it would seem that axial force from pressure would tend to expand the bushing radially (as Ed says), but that would make the hole bigger (as when expanding a nut by heating it). OTOH, radial pressure applied to the outer cylindrical surface of the bushing (whose area is much larger than the area at the end available for axial force) would indeed squeeze the grommet onto the probe. But perhaps the same pressure could make its way between the probe and the grommet, partially negating the effect? Regardless, that phenomenon doesn’t depend on the Poisson effect. The Poisson effect from the external radial pressure would lengthen the bushing though, just like internal pressure in a pipe shortens the pipe.
Not to bore, but a third possibility is that axial force on the end of the probe itself would tend to push it outward. But the drag on the inside of the grommet would cause, via the Poisson effect, an axial deformation that leads to a tightening of the grommet on the probe–like trying to pull a hose off a tube instead of pushing on its end, or Chinese handcuffs. Seems like there’s not much force involved though, since the area is so small.
Interestingly, cork, unlike rubber, is one of few materials with zero Poisson effect. If you start to cork a bottle of wine, squeezing the cork, the part of the cork not yet inserted does not expand or lengthen. Not so for the rubber stoppers now in use. IMHO.
Jaguar didn’t invent this thing. It was an established method of installing sensors and the like for many years. I would hate to have been the engineer proposing the idea in the first place, but once established it was simply a matter of “how it’s always been done.”
That said, perhaps it’s worth noting that Jaguar did away with this push-in scheme for the fan control switch and went to a threaded fan control switch. Anyone with the earlier press-in switch can either tie some wire around the water pump inlet elbow or simply buy a new elbow with the threaded switch. Almost nobody who knows the thing is there wants to drive around wondering when that rubber grommet will harden just enough to let the fan switch blow out and blow coolant all over the highway.
The same “top hat” seal for the push-in fan switch is also used on the coolant manifold pipes. This is a more suitable application, though, because pressure is applied equally at both ends of the pipe so there’s no net force trying to push it out. It’ll stay put until the top hat seal crumbles or the pipe itself rusts away. I replaced my coolant manifold pipes with copper pipe; others have gone with SS or brass, any of which mean you can forget about leaks there for decades. Dielectric corrosion isn’t an issue because this pipe is electrically isolated by the top hat seals.
Finally, note that the condition of rubber parts such as these isn’t simply a matter of age. It might also depend on how hot the engine has gotten. If your engine has been overheated, it might be a good idea to just go ahead and replace all the top hat seals – along with the timing chain tensioner which also doesn’t care for heat, and every O-ring you can get at. In fact, it’d probably be a good idea if someone came up with a checklist of all the things to replace if your engine has been overheated.
Kirby, I have to admit that I did experience a failure of the aux fan grommet. It was after an overheat situation, and I don’t know if the grommet had ever been replaced. Either the switch popped out of the grommet or the grommet out of the housing, but it was a total emptying of the coolant on the garage floor! I bought a used housing that had the threaded switch, but never got around to installing it. That was many years ago, and before I had gotten my cooling issues sorted. It is still in place and to my knowledge leak free.
I think I may be about to reach a minor zenith in my insignificant wobbly trajectory through our space/time continuum. I am about to be pedantic in a foreign language…
All this Poisson talk is a little fishy. Should it not be “Poisseuille”? If not, just substitute nadir for zenith - it’s all relative.
I agree !!! Just looked up “Rube Goldberg”…American cartoonist famous for popular cartoons depicting complicated gadgets…although I don’t think endless hours were spent on the coolant level probe…but some of the other stuff certainly qualifies…
The coolant probe seal is a moderately critical detail which completely relies on the condition of a rubber bung to work. Rubber deteriorates over time so this detail IS going to fail…and mine was weeping…as a precursor to popping the pressurised and partly lubricated pin. There are plenty of standard and proven methods for permanently sealing this type of connection …you could possibly check a modern Japanese car for some examples…but I always improve my Jag whenever I encounter a fault like this…so this pressurised tank seal is on the list.
Pete
Definitely “Poissons Ratio”…Concept being stretch a fish to make sure it is long enough to be a legal catch and it will start to resemble an eel!!!
Matt
Ah! Ratio. Very clear now, merci beaucoup. Maybe once it starts peeing out Poisseuille would have a finger in the pie (or grommet)?
Nadir it is then!