EvolvnJim

My 2014 Mercedes ML350 Bluetec with 253,000 miles was having coolant problems and leaks. Mechanic replaced the coolant reservoir and a cracked, black plastic elbow deep against the firewall. The SUV ran okay for a week but at highway speeds was starting to overheating after an hour. Gauge red-lined then dropped briefly to normal for a second then shot back up. It was then driven off the highway (2 miles) to a safe exit to cool down and then tow back to the mechanic.

At the mechanic today he showed me the coolant level which was fine but the color of the fluid in the tank was black! They can't explain why. I asked while they try and figure that out I asked to have the oil & filter changed (it's close enough to due). I am interested to see if they detect any discoloration in the oil that drains out. Don't know what to do next. Any ideas? Is my SUV now shot?

Please advise, Jim