Originally Posted by lucid
Strangely enough, not an issue at the Glen, but this was a consistent issue at NHIS last year, which is a much slower, but tighter and technical track. I don't have a good handle on what contributes to the overheating of the PS fluid. I don't know where the energy input into the system is exactly. Does it have to do with the steering input and how hard the pump works? And maybe on a faster track, there is more airflow into the engine bay and hence more cooling for that system?
The E39 M5's had this problem until someone solved the problem by taking the level in the PS fluid reservoir down about 1/2 inch to make more room for expansion.
The PS fluid doesn't "boil" or "overheat". It just expands as it gets hot and when all the air is pushed out of the reservoir then some fluid leaks out. It's that simple.
One thing that does seem to make the fluid hotter is bypassing it under pressure, which is how the less boosted "sport mode" steering is achieved in MDM. I run mine as standard power steering boost - the wheel's lighter but the fluid's cooler.