Hey guys!

I've got an SL500 MY05 (M113) that no matter how hot it's outside it keeps the coolant temperature at 90ºC sharp, no fluctuations even if idling for a long while.

However, on my SL350 MY04 (M112) I used to see the coolant temperature dropping to 80ºC at highway speeds and reaching 95ºC when idling and during hot weather low driving speed too. I thought I could have an issue with the thermostat and got it replaced a few days ago. I've noted since then the temperature does not drop that much at high speeds any more, however it's still hitting 95ºC while idling or driving in temperatures above 30ºC.

Any idea what the issue can be that the needle is fluctuating on the 350 but stays at 90ºC sharp on the 500? Any software change between these two MY?

Many thanks!

 

Any help guys? Anyone can comment if their SL coolant temperatures fluctuates while driving? Thanks

 

It seems likely that your new thermostat is programmed a little differently, as long as it is staying about 90 you should be good. Assuming that your new thermostat is a good quality product. On the cooling systems on my R230s I would never use anything that's not genuine Mercedes parts. These cars are the wrong place to use cheaper, off-brand parts. Also assuming that with the thermostat change, new coolant was used and it's the correct type. Make sure that it is!
If not exceeding 95 at ambient temps of 30C (86 F) that is within range...if it went higher than that, you could worry but 95 at high temps is normal. If you don't trust what the coolant temp sensor is saying you could replace that. If you decide to do that, I would get a genuine Mercedes part from FCPeuro with a lifetime guarantee for only $26.50 Genuine Mercedes 0009056102.
It's pretty unlikely the temp sensor is reading incorrectly, however, the new thermostat is probably just tuned to stay at 90 "all the time" and excess heat like 30C while idling or sitting in heavy traffic on a very hot day, will result in temp rising to 95. I don't think you need to worry.
Years ago, I had a Chevy and for those you could buy different thermostats, for varying temps, and I knew a mechanic who had his own shop for decades and he told me that "200F is a good running temp for engines because they stay cleaner inside at this temp" and 95C is 203F. So if 95 is as high as it gets, and actual normal running temp is 90C, (194F), these are certainly acceptable numbers. I had a Buick Riviera in the 1980s that would reach 240F (way too high) I installed a temp gauge on the dash because all it had was an "idiot light" that comes on after the engine's melted. It would only hit 240F going up a mountain, after crossing the peak, going down the back side, it would cool down again to normal temps very quickly. 240F being 115C, now that is really hot. Today I would not find that acceptable!

 

Of little help, but mine fluctuates like yours 2003 sl500

 

So does mine; always has. Sounds normal. This issue was corrected on modern cars with the deletion of the temp gauge. What you can't see can't hurt you...