Whats an expansion tank????

[Booth_S4]

Pulled out the documents for my S3 the other day just to have a nosey and sort them out abit!

Found a receipt for when the new engine was put in, it also had the revo and diverter valve fitted aswell, but noticed there was also an expansion tank on it!

What is this and what does it do??

thanks

 

[Caesium]

the only expansion tank normally associated with an engine is the coolant epansion/header tank, where you fill up with coolant.