Mercedes states that their cars meet 'specs' at 80C min. I'm sure your car has the same spec. At 80C and above it should meet emission, and all other specs.

The fact that your car is running 90C on just water most likely means that something isn't right. By all means, do the flush (that's some flush as detailed by bhdoctor) and see what happens. What have you got to loose but some time and a few $$.

Looking forward to how it works.

to post an update, I have gotten to the part where I have used the degreaser (shout) and flushed it 3 times from the system. The car seems to run a bit cooler, but still not cool enough. I will try the citric acid flush. I haven't done it yet as it's too wet outside, and I live down 7 miles of gravel and dirt road to get to pavement. I don't feel like washing off the underside of the car each time I empty the system.
I think I will have to pull that radiator out and clean out the fins (cores, whatever the small thin pieces of metal on the outside are). They look pretty clogged. As well, clean out the outside of the refrigerant rad.

Please let us know what the outcome is. My 280CE will run up to 100C in traffic but come back down to about 90C once I start moving.

I think I might have found out why the car is running hotter than normal. I was looking, and Arizona ******** advertises the tstat as being a 87 degree C tstat. http://www.***************/search/product.aspx?sid=3rigz245usuvqq453ocnzsns&makeid=800016@Mercedes&modelid=1195657@S320&year=1995&cid= 16@Cooling%20System&gid=2200@Thermostat It would explain why the car's running hotter than I think it should. I flushed 2 times with citric acid so far. The temp isn't remaining stable (running plain water could be the cause). It's varying from about what I would think is 87-97C. It hasn't crossed the 100C mark since I started flushing. I still have to flush the citric acid out of the system, but the car's too hot.
I'm still thinking that I might have to pull the radiator anyways and clean out the heat transfer fins. It doesn't look like it's ever been done. The top bolts that hold the rad in are still covered by untouched factory paint. So, maybe tomorrow.

Changing the tstat to 80C will help. . . but not much. I did that to my car 5+ years ago when the old tstat was found to be bad.

With plain water (and some citric acid), the engine should run as cool as it EVER will . . . since water is what cools the engine.

So you are correct to remove and clean out the radiator's fins. If they're blocked, the rad can't cool the coolant since there's no air flow through the radiator.

If the radiator is still 'good', that will make a big difference. If you want to 'test' it, bring it to a rad shop and have them check it out. While it's out, is the time. . .

before you did the citric acid flush did the temp get above 100C.

79 Mercy, that's a two pronged question.
Before I changed the tstat, it did regularly. I thought the operating temp was 102C. (Which with a -40C or lower winter, I didn't mind, I might add. Never ran out of hot air for the heater.) Then the temp started climbing slowly, until I'm sure the tstat wasn't opening at all.
After I put the new tstat in, it would get very close on just water, but never crossed. We're talking about maybe 2 degrees under. The I flushed twice with citric acid, and the temp has for the most part stabilized at about 90C, although when I drove it, the temp would fluctuate between 87-97C depending which way I was driving. (This due to wind) It has me a little concerned about having to do a head gasket job. But, tomorrow, I'll yank out the rad, and clean out the heat transfer fins (anyone know what they're called??), and see how it works after that. I should then rinse it out about 3 more times. I'm not seeing any rust flakes, or sand coming out. Hopefully, that's my only problem. I have everything apart right now, so might as well do it. I'm lucky I have other vehicles to drive in the meantime.

I took the rad out, and cleaned it. Between the AC rad and the dngine rad, there was a lot of caked dirt and hair, and leaves, etc. I'm surprised that the car didn't heat before this time. I cleaned it all out, and replaced the water pump which was leaking.
The citric acid flush worked well, but didn't lower the temp much. The engine just cools faster now. The specs from the mercedes sheet says that the tstst starts opening at 87C and fully opens at 97C. I have been fooling around with the car to see what happens, and it doesn't get above 100C. It will cool down to 90C while driving down the highway at 30C outside temperature however. In stop and go city traffic, it runs about 100C.
I filled the car with Zerex G05 coolant, and distilled water at about a 60/40 mix. While I had everything apart, I replaced the ilter in the power steering. Seems like it runs good.
Thanks to all those who gave their advise.
I had to buy a special socket from snap on to get the pump off however. If someone is having the same problem, the socket number is TUAM6E.
When working on an alumanum engine, it's really important to get all the torque values right.
That's all for now folks.

60/40 . . . do mean 40/60. . af/water? 60% af will do nothing except allow heat to stay in the engine. Now 60% water (a difference of 20%) will make a difference!

Jim, It was 60% af as in winter here it frequently gets below -40C. I have seen engines register at -55C on an infrared thermometer.

<... down to 90C while driving down the highway at 30C outside temperature ...>

but it summertime . . . when winter comes you take appriopriate steps to winterize your car.

Practically that's also easy to do; siphon the contents of the over-flow bottle and replace with distilled water.

When fall arrives, you do the same thing but replace w/ af.

Since it's "30C outside temperature", the 60% af does nothing but heat up the car!

Common practice to have two mixes when the climate temps are seasonally extreme..
I lived in the N.E. most of my time and it was standard procedure on all the vehicles , specially the tractors and equipment..

When I was a kid, we used to have a Winter thermostat and a Summer thermostat... you changed them at each season and threw the other one in the glove box for next season....