Are these standardized, meaning I can pick one up at any auto supply store, or do I have to order one for my car from the dealer. Thanks.

 

RockAuto.com has the complete tank assembly with cap. Fairly inexpensive.
correction: They do have the cap alone their website also

you might want to get the whole tank if only as a backup. They do crack.


Z

 

I picked one up from Advance Auto that worked - but it only lasted about a month before it came apart - so pick your poison

 

Thanks all - 'found one at SNG Barratt. p/n: MJA4440BA#

 

The OEM caps are indeed better quality than the aftermarket ones. I have tried both over the years. The longest an aftermarket cap has lasted for me is about a year. My wife's 2006 XK8 is currently running a September 2017 aftermarket coolant reservoir from rockauto.com with a November 2015 OEM cap that I purchased online somewhere else....