The brief
Mr Li, a regular of ours, brought his AMG GT in down on power, and a 360-degree check on the lift plus a look at the oil told the story, coolant had got into the engine oil. Oil and coolant mixing is one to take seriously, because the two are meant to stay apart, and once they don't the engine isn't being cooled or lubricated the way it should be. On this engine the oil cooler runs engine oil and coolant past each other so the coolant can carry heat away from the oil, and it seals between the two sides with gaskets and seals. When those seals fail, the oil side and the coolant side join up, and because the oil sits at higher pressure it pushes into the coolant, or coolant gets drawn into the oil, contaminating it. You get the cooling and the oil both upset, and the engine running poorly. Failed seals don't reseal themselves, so the fix is to renew them and clean both systems out.
The diagnosis
The check confirmed coolant in the engine oil and traced it to failed seals on the oil cooler, where the oil and coolant sides had joined, which is the contamination and the power loss. The head gasket and the block checked out, this was the cooler seals, not something worse, and the engine itself was sound. That's a reseal of the oil cooler, 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 oil cooler was opened up, the failed seals and gaskets replaced with genuine Mercedes-spec parts and everything 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 coolant in the oil.
The outcome
Full power back, a clean idle, the temperature gauge steady, oil pressure where it should be, and the oil and coolant clean and separate again. Mr Li got the AMG GT back quickly, running properly. Catching it as failed cooler seals 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.