With the coolant/water the temp will be lower than air because air that is compressed or under pressure is hotter. Go to FCP euro and type in your car year and model then search for the thermostat. Even if it's the same one 84*C click on that and then scroll and look for the fitment section. Then you will see several different years and models that all will use that thermostat. Then what you do is just a reverse search for one of the models listed that use that thermostat and see which model offers a lower temp thermostat. I believe I saw a 1981 380 SL as using the same thermostat, but I'm not sure. Anyways, go check it out. Good luck.

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1990 190E 3.0L