The temp is running right, it should be over the 80 mark in normal driving, however you should not be losing coolant. These systems are full recovery and will not use or lose any coolant in normal driving. If you have over 100k, check the water pump, those usually go at about that mileage. The resevoir tank is on the fender and can be checked when cool. My car did the same thing, radiator light came on while in city, went out on the highway, My water pump failed. When the bearing fails it will leak from underneath from a little hole, if you have MB coolant it leaves a white residue when dry, take a look in there and you are sure to find where it is leaking by locating the white streaks. If you don't have MB coolant it will be harder to find, but be sure to put the right coolant back in after the repair, MB coolant is specially formulated for the aluminum alloys in the engine. Keep an eye on the temp until you get it fixed. Enjoy your car ! the 140's are a wonderful automobile!
Regards,

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Brian W. Heitman

www.MBCA.org/northtexas

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