The brief
Mr Li, a regular of ours, brought his AMG GT in down on power, and a look in the coolant told the story straight away, oil mixed into it. Oil and coolant finding each other is one to take seriously, because the two systems are meant to stay separate, and once they don't the engine isn't being cooled or lubricated the way it should be. On this engine the oil cooler is a heat exchanger that runs engine oil and coolant past each other through thin walls so the coolant carries heat away from the oil. If one of those walls fails internally, the oil side and the coolant side join up, and because the oil is at higher pressure it pushes into the coolant. You get oil in the coolant, the cooling and the oil pressure both upset, and the engine running poorly. A failed oil cooler doesn't repair itself, so it gets replaced and both systems cleaned out.
The diagnosis
The check confirmed oil in the coolant and traced it to the engine oil cooler, the internal heat-exchanger wall failed so the oil and coolant sides had joined, which is the contamination and the power loss. Crucially, the head gasket and the block checked out, this was the cooler, not something worse, and the engine itself was sound. That's an oil cooler replacement, plus a thorough flush of both the oil system and the cooling system to get every trace of the mixed fluid out, and fresh oil and coolant.
The work
The failed engine oil cooler was removed and a new genuine Mercedes-spec cooler fitted with fresh seals, every fastener torqued to the manual figures. The oil system was drained and flushed and a new oil filter fitted, the cooling system drained, flushed thoroughly and refilled with the correct Mercedes coolant, and the engine refilled with the correct AMG-spec oil. The system was bled, pressure tested, and the oil pressure checked good. A road test confirmed full power back, a clean idle, the gauge steady, and no sign of oil in the coolant.
The outcome
Full power back, a clean idle, the temperature gauge steady, oil pressure where it should be, and the coolant clean with no oil in it. The AMG GT went home running properly again. Catching it as a failed oil cooler rather than something worse, and flushing both systems out completely, meant the contamination was gone for good and the engine was protected the way it should be.