Does your water temp move at all on track?

[shane@mbtech]

You theory is correct mate.
However, I have seen many times in Greece (with temps above 40C) BMWs having overheating issues when driving up a hill/mountain on 2nd or 3rd gear (40-80km/h) which were caused only by the faulty viscus fan.

You will notice the sound of the fan on a very hot day when accelerating on 2nd gear for example.

Yes, but it does not mean its doing anything. It just means the temp in the block is enough to kick off the viscous.

How can a viscous fan, which is only pulling outside air (40*c) through the rad exceed the effect of the air rushing through at 50mph? And stop it overheating?