The engine temp gauge on my 1999 SLK230 ordinarily sits on "80" when up to temperature. The only time I've seen it above 80 (85) is on a long road trip in 100F degree weather.
The past few days it heats to 85 and shows signs of going higher, towards 90 after driving it around town with the outside temperature in the mid 70's. I also notice that the engine comes up to temperature much faster from a cold start than it used to. At what temperature should the radiator fan come on? I've never paid any attention to it in the past. The fan never seems to run. With the engine off I tried turning the fan and it is solidly stuck. Seems like there should at least be some play in it. Thanks for any advice.
Did you check the electric fan or did you check the viscous fan clutch (if installed). The fan clutch should move but move with some sort of resistance. If the fan free wheels (when it continues to spin and not appear to stop when turned by hand) then this could raise the coolant temperature when the vehicles engine is running and the car is parked or stuck in traffic.
My concern is that you said you can't move the cooling fan, it should move freely.
If it doesn't move at all you should bring it into the shop immediately to have a professional mechanic diagnosis the fan to make sure it is not seized.
If the fan checks out to be good then you should check the coolant level making sure it is filled to the proper level.
The thermostat would be the next item to be checked for proper operation.
If the problem still cant be found then a mechanic can use a scanner to check the electrical components of the cooling system to make sure they are operation properly.