Has any member found a reliable, long-lasting expansion tank cap (4.2 Liter or otherwise) that will hold the correct 15 psi (or 100 kilopascal, if you prefer) for any length of time? Or know how to disassemble and free up a cap? I have tried all types - oem, aftermarket, Stant, etc., While they work for a brief time, all eventually stick/slightly corrode and fail to fully allow the return of coolant from the recovery/overflow tank, resulting in the collapsed upper radiator hose phenomenon. I have all new hoses, water pump, thermo tower, etc., and know the lines are clear and correctly connected. I am not greatly concerned, as even the 15 psi pressure might be expected to slightly collapse the hose. I do think, however, that the lack of pressure equalization contributes to the common weakening/cracking of the expansion tank itself and failure of the other soft cooling system parts. Any insights much appreciated.

 

I assume the expansion tank cap on my wife's 2006 XK8 (built in May 2005) was the factory original. I noticed about six weeks ago that it was allowing a bit of coolant to seep through its seal regardless of how tightly I cranked it down on the expansion tank. After attempting to clean and lubricate it with no success, I finally bit the bullet and replaced it with a new one from the local Jaguar parts department and my leak issue was immediately solved. I paid about $17 after my discount (they long ago learned that if they don't work with me, I won't deal with them). I had read here on the forum about these caps sometimes failing, but not repeatedly as your car's caps have done. Keep us posted if you find a better cap out there....

 

I need to get a new cap also. When my serpentine belt came off 2 days ago and the engine briefly overheated it never sucked the coolant back in so it did the collapsed hose bit also until I opened the cap and it equalized the pressure and I had to top the coolant off manually.


Dave

 

I am a bit mystified by this thread. I've had Jags ranging from an X40, X300, 2 x X305 & an XK8 - and none have ever shown any sign of this problem (even when I boiled the XK8 over).

Have any of you removed/replaced the expansion tank, or changed the hoses running to it? The reason why I'm asking is because the two pipes to the tank need to be crossed over, so it looks wrong. Failure to cross them causes hose collapse, water loss etc.

 

jimforrest,

What do you mean by the pipes to the expansion tank needing to be "crossed over"?

When I replaced the leaking black plastic coolant outlet duct last November, I also replaced the brittle mostly plastic hose running from the top of that duct to the expansion tank with a much better rubber fuel-line hose. Nothing else there to "cross over" with, so I don't understand what you mean....

 

The crossed lines pertain only to the 4.0 XJ8.

 

Oh right RJ237 - there you are Jon89 (I'll crawl back under my rock then!)

 

To Jon89:


Since the cap seals from the side with O rings, not the top with a rubber washer/gasket, I have tried not turning the cap all the way down to leave space for the backflow, but no improvement.


Since the spread of the internet, pricing at our local dealer has moved off list also. It approximates repair shop pricing and is not much above internet for many parts, but always have to compare. We can probably thank Merriam, Rock Auto et al for the well-deserved price relief. Now, if someone could only do something about the labor rates ($150 here) and times.

 

May be an indication of the sensitivity of the spring, and its lack of durability under somewhat adverse conditions. Apparently, when overheated, it loses the proper temper and doesn't hold the correct pressure (?)

 

I never had the hoses collapse or low coolant condition ever until the belt came off and it overheated briefly. No coolant was lost externally, it just didn't suck it back up from the in fender overflow container after it cooled off and the hoses collapsed until I opened the cap. I have since topped it back off but haven't been able to run it again until I get the belt and pulleys and install them. So I don't know if the cap is permanently broke or if that was a one off occurrence because of the overheat. I'll replace the cap anyways I guess.


Dave

 

After three cap replacements with the same result I would think it might be time to replace the reservoir. Most come with a new cap and sensor. Really simple DIY replacement.

 

Replaced the tank two years ago because of cracking and resulting leak. The new one shows no signs of the discoloration/aging that is common for them and seems to be holding up well. It (as well as the entire cooling system) must be holding tight or, of course, the hose wouldn't collapse. Tank came with a new cap when I got it, and siphoning has worked in the past. Have probed the openings with a wire and blown air through the lines. All seem to be clear. One of those mysteries of the Jaguar life.

 

There are two valves in play here: the one that opens at 15 PSI of pressure, sending coolant to the recovery tank, and a return valve that opens when there is vacuum in the expansion tank, below the main seal. How much vacuum, I dont know, but surely no more than a few PSI.

For the hoses to collapse, I think two things have to be true: the main seal in the cap is good, and the return valve is not opening (or is clogged somehow).

 

May have found part of a solution for my expansion tank cap not allowing the return siphon of coolant from the overflow tank. Have been lubricating the O-rings in the cap with silicone grease to allow easier removal, and, since doing so, coolant has returned and remained at the top of the neck during cool down. Perhaps the rings were distorted from the tight fit and so blocked the slots in the neck that allow the coolant return. Any case, certainly a lot easier getting the cap on and off!

 

Good information to know. Keep us posted as you continue to monitor your now-greased cap. Perhaps this approach will work on other ancient caps out there. I would try it on my original factory cap but I tossed it once the new cap went on and worked as designed....

And I do indeed remember how difficult it became to unscrew my original factory cap the last couple of months before I replaced it with the new one....

 

Will do. We all need help keeping our cooling systems young and functioning (same with the drivers). Hoses/soft parts certainly the weak point of this model.