On a warm to hot (80*F and up) days my 300SD would run at 80*C on the spot, when the temperature drops around 70*F and below my car would run around 85*C-90*C, is it normal for it to run warmer in cooler climate and run cooler on warmer climate? I am not complaining here but i'm just curious.

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Current Garage
2008 Mercedes GL320 CDI 188k mi Repair/Work in progress
1994 S350 160k mi Garage Queen & prepping for repairs
2005 E320 CDI 203k mi Healthy & Daily Driver
1994 S350 357k mi Retried as parts car
1984 300TD 214k mi Blown OM617 Poss OM603 Swap??

Sold
1987 300SDL 200K+
1994 S320 181K mi
2008 E320 Bluetec 127k mi
1999 S420 130K mi
1980 240D 360k mi
15+ Others that has come, stay and gone

GONE, BUT NOT FORGOTTEN
1995 E320 SE 220k mi
1984 300SD 350k mi

Clearly, you're data is flawed. Your conclusion is impossible.

I observed mine doing the same thing today. The car ran a very stable temp just above the 80 mark for about 4 months of unbearably hot south Louisiana heat. Now we have a cool spell with highs in the 70s, and the car is running hotter than it did all summer, though not approaching an overheat situation (i.e., not reaching the next mark up). The gauge seems to be fluctuating more as well. Just a casual observation as I was driving around today, but I also thought it was a bit counterintuitive. I also noticed that the fan wasn't running today, and I think it ran constantly during hot weather. Might be just an aux fan issue.

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1983 CD
1958 Studebaker Packard (being resuscitated)

I know why my Chrysler does this, don't know if it applies to Mercedes.

When the A/C is on, the auxiliary fan runs all the time. I have a 180F thermostat, which is more than 10 degrees cooler than factory. When you aren't using the AC the auxiliary fan doesn't come on till the engine temp rises to something over 200F. So on relatively flat terrain the temp gauge stays below the half way mark at all times, but in mild weather without A/C use, the temp gauge fluctuates a lot more.

I think this is relatively common. My Lexus temp gauge used to stay in the same place at all times when I lived in AZ. Now that my air isn't working I've noticed the same phenomenon, but to a lesser degree.

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1979 240D, 4spd manual, Power Sunroof, manual windows, 147k miles, Pastel gray/Black MB Tex.
1991 300D 2.5 - Smokes like it's on Crack!

The larger variations I could see possibly being due to the cycling of the thermostat, and the cooler charge of coolant from the radiator causing the bounce on the needle.

That being said, I'd be wondering if your thermostat was starting to fail, as well.

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-Josh
Testing the cheap Mercedes axiom, one bolt at a time...