I had the same problem on the exact same coolant tank. I went to a junk yard and found one from a 2.5L. The one I got had grease all over it, so I took some hot soapy water poured it in, and shook it salt shaker until I could see the bubbles. I rinsed it out and put it on the car looking brand new, w/ the exception that it was yellow-ish.
Just do this w/ you coolant tank and you will be good to go.
Also, I had a coolant flush just to clean out the old nasty coolant.
-KJ
