Quote:
Originally Posted by patbob
... steam bubble expose the sensor...
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My guess. I doubt the engine could actually heat up so fast you see the dash gage jump from 90 to 120 C in 2 sec. But, the sensor itself could get that hot that fast. I have seen this after refilling coolant. If the sensor sits in an air/vapor pocket, it can read much higher than the coolant temperature. Add a little coolant, and it drops to what it should read.
I have driven that road to Onion Valley long ago (not in M-B). The valley floor is so sloped that, coming back, you can put in N and glide 70 mph all the way (straight road) back to Independence. Thus, your front end was tilted up, putting the sensor at the high point.
Even if your engine did briefly tap 120 C, you wouldn't do damage as might happen with an aluminum head (like to warp). You didn't even boil over, so unlikely the coolant even got as hot as the dash gage suggested.