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I've had a similar issue this year, but my aux fan does work. I can confirm Shady/others are correct in that the aux fan does more. While trying to diagnose, I drove the car for an entire day this fall (mid 60's F) without the mechanical fan installed. I had pretty much the same operating temps in slower speed driving. It gets into the low 90's C. I'm sure it would go higher if I kept idling and moving too slow to move enough air across the radiator.

Turns out my issue is the radiator, which I was suspecting even though it's only 7-8 years old. Not enough flow at low speed and idle in traffic. I had already removed it and cleaned debris at the end of summer. It helped a bit but now I have the smoking gun - the radiator started to distort - it expanded the core. Internal clogging is my guess.

Since you're going to replace the fan, you'll have a chance to review the radiator condition. If you're hitting over 100C, you might suspect more than just the aux fan. As you know, best to replace one thing at a time though.
 
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