I have a nice 95 S500 with 163,000 miles car still looks great but having overheating problems. changed mono valve, changed thermostat, check water pump ( good ) also flushed system. I was told it might be the head gaskets are bad car does not leak don't have water in the oil idles fine when you drive it heats up fast 5 - 7 miles then 120 C. If it is head gaskets how big of a job is it? Thanks, to all that can help

 

That is a major problem, even with aircon on my 97 S500 has never ever gone over 100 degrees. 90% of the time its between 75-80 on the open road (here in the Uk I don't need to use the aircon too much), and about 85 in city traffic.

My advice is as soon as it gets hotter than 100 stop right away, the aluminium head will warp peremenetly if it overheats too much!

Try changing the radiator, its probably full of crap, change the viscous fan clutch. What coolant are you using, Mercedes are very fussy, put in genuine Mercedes coolant at 50/50 and the car may then sit at 80.