My expansion tank is broken on the inside the separator broke and some where in the system I am losing coolant and sucking air. Going to swap it out for a Euro tank set up. herd its more reliable.My single vanos M52 is very low mileage for its age little over 87,000 miles on it. General consensus on the forums is that the vanos seals fail very early. Until I have problems with the vanos itself, I will not be servicing it. I have not driven any single vanos M52's with the seals changed, so I would like to see for myself if there is actually a difference in timing performance.
If your expansion tank is not leaking I would leave it alone, I decided to do preventative maintenance regarding that part. Swapped my original expansion tank with a dealer bought one, the part failed in 2 months. I could just be unlucky in that regard, but everyone states buy the tank from dealer. Well its garbage, and there are some threads where a person has gone through multiple dealer bought expansion tanks that failed. So if the original one is intact I would leave it alone.