Radiator cap? coolant leak

Jhill

It's not uncommon to see a cooling system where the reservoir is actually part of the pressurized system. 99% if not 100% of all euro cars operate this way and American is about a 50/50. As to design function vs an "overflow" reservoir the only thing I can think is they get that little bit of extra volume of coolant to use. As for why the z has two caps, pre 2012 the solid cap would have been the pressure relief cap allowing coolant to go to the overflow, post 2012 the reservoir cap is the pressure relief one and the one on pipe is solid so in a sense it is now just a pipe, save cost on manufacturing a whole new pipe.

Oh and some systems where the radiator is actually not the highest point in the cooling system they can mount the remote reservoir to a high position (typically by the firewall) so now it is easier to purge air from the system.