What's the pressure rating and condition of the coolant expansion tank cap?

I believe it should be 1.4 bar which is around 20 psi.

If it's a Mercedes OEM cap, it should be marked "14", but might also be marked "12", which is 1.2 bar and is too low. Mercedes revised the cap spec to 1.4 bar.

My 190e 2.3 had the 1.2 bar cap on it when I bought it (it had just had the head gasket replaced) and it spurted out the overflow pipe as well untill I fitted the 1.4 bar cap to the tank. Now all is well.