Update - a double whammy
Turns out the coolant cap on the expansion tank was bad, BUT I also have a leak in the neck of the expansion tank. I didn't notice the leak in the neck of the expansion tank last night because of it's proximity to the coolant cap which again was bad. After replacing the expansion tank cap it appeared that I might have licked the problem. Driving a little farther raised temps and then the additional problem with the leak in the expansion tank neck reared it's ugly head.
When a head gasket goes, engine temps run higher, water appears in the oil and you have a cloud of steam at start-up in the morning. In my case none of these have occurred. I simply get a bit of backing up in the coolant expansion tank after shutting off a warm engine - the result of a system that's not holding pressure.
Regarding the fan clutch - I am suspicious of this animal and have been for a LONG time. It seems that you read alot about MBs going from 87C on the highway to 100C in stop-n-go. I haven't heard the ROAR that used to kick in at about 102C in quite awhile. I have noticed that after shutting off the engine when fairly warm that the fan blades continue to spin freely for quite awhile. Then again, once the engine has REALLY warmed up, the fan blades stop immediately after shutting off engine. I'm tempted to replace the fan clucth, but at about $150.00 I think I may wait. I have never seen any temp > 105C and that was in 100F+. I live in south Texas.
Tomorrow I'll order a new expansion tank, etc. As JeffSr says, there will always be something else to fix.
Thanks to one and all for the advise/opinions.
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.