Thank you all for the tips, based on this and being able to stop the fan with a tube made from newspaper sounds like the viscous clutch really has failed, so that will be replaced as soon as possible.

Andrewwynn's idea to test the auxiliary fan manually sounds good, I'll test that tomorrow with INPA. My guess is that the auxiliary fan is shot too, and in that case I'll replace that too. Water pump was just replaced and at least externally the radiator looks fine.

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09/2003 X5 3.0i
02/2003 X5 4.6is (non-runner)