Drop in temp has to be map tstat related. I think it just caught you by surprise. The relatively cool ambient combined with the computer deciding to cool off.

It's very dramatic on the n55 motor. When tstat opens it also kicks the coolant pump 67% faster and ECT temp drops from 109 to 95 in well under a minute maybe half.

This will happen any time you hit a hill and engine develops load. If it's a small hill, temp might only drop to 100 or high 90s but even just a bit of throttle to overtake and temps will drop 5-8c.

On n62 with standard coolant pump the change might not be as fast, but with big ass radiator just need water flow and it'll cool shit down right quick.

I'm sure you always have ac on but if it cycled on right at the same time, I'm sure the parameters are set to keep things cooler when AC running.

Interesting about no aux fan.

My car only has aux fan.


–awr–

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Every E53 should have front pusher aux fan. I have also removed my viscous on the M57. No need for it and helps maintenance.

The DME/MAP thermostat controls the temp for emissions so that might make more sense when looking at what is going on. I don't think you have a problem though.

As hot as ambient is there, 105 is a shit ton hotter than even 45-50 so there's plenty of room to work on cooling!

I'd still be a little nervous no aux fan. How'd you get AC to function? AC does a fan check at launch and disables when no aux fan.


–awr–

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two fans, two temp gauges
 

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.