Has anyone ever had problems with the cap on the expansion tank leaking? I've recently had coolant leaks and tried to find out where from as its been spraying around the top of the engine. Running the car at temperature didn't show the leak, it only happens when driving. Today, after cleaning it all down and taking it for a drive, I opened the bonnet afterwards and it was evident that the coolant had sprayed from around the expansion tank. I changed the tank (and cap)a few months ago at the same time that I replaced the radiator. These were all new items. I have ordered a replacement cap to see if that will change things. In the meantime, I have added an O ring seal inside the cap too see if that cures the problem.
Regards
Al P

 

These caps can indeed fail. I had to replace the original cap on my wife's 2006 XK8 several years ago, and had to replace the entire coolant reservoir last September. The entire coolant reservoir in my 2005 S-Type was replaced way back in September 2009 and I am pleasantly surprised it has held up this long. Regardless of where you source them from, these plastic components are of dubious quality and I quickly learned to check their condition every weekend as part of my weekly fluids & pressures check....

 

Yeah, the caps can fail....but I'm dubious that's your problem. Look closer....likely leaking from the nipple under the cap being cracked at the bottom. You don't bother to tell us what engine you have...but if the 3.0L, it's a good bet. If a V-8? I've no idea if they have a plastic nipple molded onto the filler neck and the whole arrangement placed in an ideal area for an unaware tech or casual observer to lean on it whilst doing something unrelated under the bonnet.

 

Mine is the 2.5 V6. After driving it yesterday, I noticed what appeared to be a spray pattern from the cap. Today after the drive to work, I checked to see if the O ring seal had worked and it had, no coolant leak at all. So will replace the cap with the new one when it arrives.
I had heard of problems with the bleed tube attachment, but my leak is not from there.

 

I am buying a new expansion tank cap tomorrow for the same reason.
Its been dribbling out coolant from the underside of the cap onto the top of the expansion tank for a while, not major just unsightly.
A new screw cap is £10 on eBay uk.