Originally Posted by jml
I'm not convinced it is boiling since there is no indication that there is cavitation in the pump. Unlike the E46, the E90 has a PS oil cooler. It's probably a slight expansion in volume due to temp, and an increase in pressure due to sustained high RPM and high temp.
I thought the issue was that the pump is right next to the headers. So extended track use heats up the headers which increases the p/s pump temp with also increases the p/s fluid temp.