When I took over the care of my dad's 87 300E, I took it to my mechanic for update on any maintenance neglect. He found no records of the coolant having been changed in 6 years! The water pump was starting to leak, and the hoses were bad. He replaced the coolant, thermostat, plastic 'stat cover and hoses immediately, and a week later replaced the coolant again when he did the water pump. The temp always ran in the high 90's before the changes, and now routinely runs in the low 80s.
87 300E 53,000 miles