The OEM themostats are designed to BEGIN opening at about 87C, but there is several degrees tolerance to this and they can drift over time. My '88 190E 2.6 used to run 90 on the freeway in nearly all weather. Now it runs 80 at up to about 70F ambient and 90 if it gets up to 90. I not worried about this behavior.

The purpose of the thermostat is to regulate the lowest operating temperature. The highest is controlled by the fan algorithms.

In traffic in hot weather it can get up to 100 or more. After my last emission test, I started up the car the auxillary fans engaged at high speed and the engine driven fan was roaring. It quickly cooled once I started driving. If you shut down a hot engine at, say 100, and start it up a few minutes later it can be 105 or higher. This is due to heat soak. The residual heat in the engine will continue to heat the coolant after the engine is shut down, but soon after you start the engine the water pump, fans, and radiator will quickly reduce the coolant temperature.

Engine temperatures between about 80-110 are normal depending on ambient temperature and how fast you are driving. When the auxillary fans engage at about 105C the engine should radidly cool unless it is very hot, like summer in Phoenix.

Radiators lose heat transfer capability as they age. The best way to keep this degradation to a minimum is to use MB Antifreeze, which is available commercially as Zerex G-05, or use Dexcool. Green antifreeze will cause radiator deposit buildup and more quickly reduce the heat transfer efficiency of the radiator.

As cars age operating temperatures may differ under the same conditions as when the car was new. This can be due to "drift" of themostats, temperature sensors, the gage inself, and the slow degradation of the radiator. No doubt the alignment of the planets in the Zodiac is also a partial determining factor.

Normal operating temperature is 80-110. Freeway driving in cool weather will keep the temp near the bottom of this range. Low speed driving in hot weather may cause it to go to the upper end of the range. If the car consistently operates at the high end even in conditions were it should probably be lower then one should verify coolant level, radiator cap pressure holding capability, and fan function, and if these all check out the radiator is suspect.

It sounds like replacing the thermostat solved your problem.

Duke