Prefered temperature while underway?
 

I have just purchased a 1980 300sd with 113,000 miles on it. I also have a 84 300sd with 250,000 on it. The 80 runs at 100 on the temp gauge where the 84 runs at 80 on the temp gauge. Outside temp was 73f today so not to hot. I am in SO fl and this is always a concern to me. I wasn doing a steady 50mph for at least 10 miles and the gauge went from estimated 98 to 102. I am not converting so that is what the gauge reads. I read in a thread on this forumn that 100 is prefered. Since for the lst 6 years I have had 80 as my benchmark is this going to cause a problem for this car. I have a 80 degree thermostat in the garage but if I do not have to install it would not bother me. Idling in the shop doing a trans fluid and filter change it registered just over 80 for 15 to 30 minutes. When I took it out on the road is when the temp went up. My wagon and coupe all run just over 80 also. Could I have a defective thermostat or is a 100 degree rating also a thermostat ?

for the 617, I believe 85-90c is in spec. 100 sounds a little high to me. I would change your thermostat first, but keep in mind that (based on my experience), they usually fail in the open position, making the engine actually run a bit cooler rather than hot. Don't quote me on that, though.

My 1975 240D ran at 80.

My 1981 300D N/A ran at 80.

My 1990 300D - Turbo runs at 80.

All temps are higher during very hot weather with AC and/or extensive climbing of steep grades. I don't think they ever got above ~90, even in the most extreme case.

Now my E430 when the fan went through the radiator. Then the temp went up a bit!

Between 80 and 90C. Is where you want your temp. 100 is getting a little hot. Your t-stat could be stuck partially close, or you could have some buildup in the system not allowing proper flow. I too am used to the t-stats failing open causing low temps issues. Also make sure all the fins on the raditor are clean and straight. Not enough oil can also cause temps to creep up.

Mine runs around 82c in the winter and 85c in the summer with the A/C going and on a hot day. I think a coolant flush and fill with G-05 Zerex and a new thermostat and gasket would be a good place to start for your car.

You should also consider that your car's thermometer is not necessarily an accurate instrument. Mine jumps up and down about 10 degrees with an obvious electrical connection problem.

my 87 runs at 84 and my 95 Gasser runs at 85..