I have a 2015 C300 Sport 4Matic with the 2.0 liter M274 engine. The car was overheating under high load conditions, with running the cabin heat making the overheating significantly worse. That's counterintuitive, rules out radiator blockage as the cause. Also thermostat failure wouldn't explain why turning off the cabin heat would cure the overheating condition. It turns out that Mercedes uses some kind of adjustable flow water pump to let the engine heat up quicker when cold by reducing coolant flow until things warm up. Great idea for raising efficiency, but when the flow adjustment piston gets stuck, you end up with this issue. Here's a quick video going over what I wish I knew before attempting this job, like that I should have bought the turbo cooler line O-rings in advance, or that the turbo doesn't need to be moved.
