bottle in RR corner S4s - what is it?

[bojangles]

just cleaning the luggage area, and noticed that bottle there, what is it for? has a pressure cap on it...
funny..

[Muck Savage]

Coolant expansion tank. Holds overflow from the cooling and chargecooling system. Should be a little bit of coolant in the bottom, if it's at the right level.

michael

[bojangles]

OOPS left rear corner.. and i thought the s4s had a shared system with the rad...

hmm it is empty.... maybe that is why my cooler is hot..