2006 XK8 with 120,650 miles....

We have owned this vehicle since early February 2012 (purchased at 36,000 miles) and during that timeframe I have replaced the coolant expansion tank once due to the typical leak at the plastic bottom hose connector (in September 2017) but I have just ordered what will be the fourth coolant expansion tank pressure cap. I have tried the OEM cap from Jaguar. I have tried the aftermarket cap that came with the aftermarket tank. Regardless of the source, all of these caps have begun to fail between two and three years of age. You know the cap is failing when it becomes very difficult to unscrew and the coolant begins to accumulate in the atmospheric recovery bottle because it can no longer return to the coolant expansion tank as the system cools down after driving....

If the $25 OEM caps fail at approximately the same rate that the $10 aftermarket caps do, I guess the best plan is simply to keep ordering a $10 aftermarket cap every couple of years or so, much as we have to do with the two VVT O-ring seals....

Has anyone had better luck with caps that last significantly longer than this? If so, where did you source them and how much did you pay for them?

 

Just ordered a new cap for my car . Losing a bit of water and noticed anti-freeze around the cap plus I was finding it hard to get on and off.Hopefully this will sort the problem out

 

They warp.

Just replace it as needed and move on to the really complicated issues that will undoubtedly arise.

Z

 

The overflow tank needs to be half full and the expansion tank at the top for it to work.

 

My latest cap has lasted 4+ years / about 60,000 miles and is still OK.

When the original cap got too warped to unscrew by hand I just bought the whole tank replacement from RockAuto.com as it came with a new cap. I figured I could always use a spare tank.

the tank is still sitting on my part shelf awaiting the failure of my original tank, which shows no sign of giving up the ghost anytime soon.

So, for what it’s worth, the RockAuto caps last as long as any I’ve seen.

Z

 

Examining the tank and cap, it appears the threads only hold the cap to the tank. The seal to prevent pressure loss is made by the long neck and O Ring that mounts into the tank. We just snug the cap down and then back it up a bit to align the message on the lid. no problem with holding pressure.
Our 2 cents worth.

 

Certainly the o-ring(s) are doing the sealing. The cap only needs to be tight enough to keep the o-ring from having room to “move around”.

this may be different on different years; my 2002 tank cap has two separate o-rings, of different diameters.

Once the cap has warped, just changing the o-rings does not change the difficulty of removing / installing the cap (I tried).


Z

 

I keep a new cap on my garage shelf at all times. Once the current cap becomes difficult to unscrew first thing in the morning when the car has sat overnight, it gets tossed and the new cap takes its place....