I can't imagine driving my M54-engined X5 in warm-hot weather, while in traffic, in N.Texas, without using an Aux fan. When my Aux fan failed, and I had a short drive to complete without it, I thanked my stars that I had previously installed a 3000cfm aftermarket electric fan (replacing the viscous stock fan). I normally have the aftermarket fan set to run at 75-80% in traffic (0-25% at 70mph on the highway), but when the Aux failed, I needed to run it at 100%, with the A/C on (but not functioning because the Aux fan failed) and in 100F heat.

I replaced the bad Aux fan ASAP, not daring to drive it without one in place.

I have two old pickups that still have their original viscous fans, but I added an aftermarket electric fan to the '98 GMC, as an aide for the hotter months, and for the A/C. Plus, it'll keep the engine coolant at a reasonable temp if the fan fails while my wife is driving it (hopefully), just long enough to get home. I haven't installed an electric supplemental fan on my '04 Chevy 2500HD, yet, but I at least carry a replacement serpentine belt (and tool), in case of failure away from home. I check the viscous clutches on both at least once a year, though neither gets driven 500 miles per.

Those trucks, with big radiators and iron blocks, and having iron heads on the GMC, and (despite having) aluminum heads on the Chevy, can withstand having a fan failure and coolant spike better than an all-aluminum BMW engine. That's why I MUST have two fans on my X5 (and a second, more accurate than the instrument on the dash, temperature gauge). Call me chicken.

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01 BMW X5 E53,3.0i-5L40E, 7/13/01
topas-blau,Leder-grau,"resto-project car"

Here:
14 Lexus ES350,3.5L-U660E
09 HHR Panel,2.2L-4T45E
04 Chevy 2500HD,6.0L-4L80E
98 GMC Sierra 1500,5.7L-4L60E

Gone:
66 Chevelle Malibu 2dr ht.,327>441c.i.-TH350>PGlide/transbrake
08 Cobalt Coupe,2.2L-4T45E
69 & 75 C10s,350c.i.-TH350
86 S10,2.8L-700R4
73 Volvo 142,2.0L-MT4
72 & 73 VW SuperBeetles,1.6l-MT4
64 VW,1.2l-MT4
67 Dodge Monaco 500 2dr ht.,383c.i.-A727
56 Chevy 210 4dr,265c.i.-PGlide