Another $0.02 worth ...
My 350 SDL just went through a bout of running hotter than normal after I changed out the coolant pump and the viscous clutch ... both had developed a bit of wobble from shot bearings.
Normal fill with 50/50.
Immediately following this I noted the temp rising to 105C very quickly - actually running over 100C while cruising at 75 mph with AC on ... it had never done this before! So, at this point, what had changed?
1. Coolant mix - yes, but how far off do you have to get with ratio in order to account for this much change? i.e. what if you ran 100% coolant ... would it run 100C instead of 82C in same load?
2. Viscous coupling - yes, but it was new ... possible to get a bad one? Yes, but mine was still running hotter than normal at high speeds with lots of ram air.
3. Coolant pump - yes, but if it wasn't leaking, and the impellers were close to the same, it would not be likely.
Baffling. So, I put it off for two weeks - couldn't stand it any more.
This weekend I changed out the viscous coupling again and changed the tstat, too. Old tstat opened and closed when boiled on the stove - rather an ancient test that I understand is not suitable for this tstat.
Am now running the same temps as before ... at 75 mph on 98F day (it was hot today) with AC on. During high load conditions, like going up long grade or in stop and go traffic at 98F, it would predictably creep up, but never hit 100C (during last hotter than normal period prior to change of viscous and tstat it would roll right up to 105C in 70C weather with AC). Something else I noticed, but did not expect, is that following work this weekend, it now runs to 82C much faster, usually within a mile after crank up in mid-60F temps.
1. Viscous clutch - yes, but same but as before. I would liked to have waited and done this later, but once I dumped the coolant, I didn't want to waste another load of MB coolant. If it had been the viscous clutch, wouldn't the car have behaved normally with ram air flow?
2. Thermostat - yes. I believe this is what failed. Question is - why now? Was it related to having drained the coolant completely? Is there a chance something occurs to an old tstat when the coolant is left out for a few hours or days? Don't know, but this is the only rational explanation.
Good luck on the 560. I would:
1. Change the tstat and see what happens. Run water after you do it in case it isn't the problem. Don't run the water long term, but a few days likely won't hurt too bad.
2. Not change the coolant pump - pretty binary with these, I think.
Please keep us up to date - I KNOW these cases drive ya nuts!
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg