About a month ago it started getting cold enough (in Florida) to notice that the operating temperature did not get up to normal to be able to heat the car, so I changed the thermostate (This is for a 1980 300SD). It is heating fine now, but the temp gauge reads anywhere from about 90 to 100 degrees. The gauge is not erratic. There seems to me no consistency between highway and local driving to the gauge reading. I start to get concerned when it is riding around the 100 degree mark. I believe that the radiator was replace a while ago, but I am not sure. Is this just normal variation in a car of this age or is this something to be concerned about and what would I need to check or what are some diagnostic steps?

I really appreciate the feedback on my previous posts and I too am becoming addicted to the site.

The thermostat on an SD has an arrow and has to be clocked a certain way. If you don't, you run the risk of engine overheating.

So, how is it supposed to be clocked?

I have the arrow pointing back toward the engine.

The arrow should point straight up.