The brief
Sam's S5 came in with the check engine light on and a loss of power, and he'd caught it just in time, while the light was still showing rather than after something worse. On the lift the cause was plain, coolant had got into the engine oil, the tell-tale coffee colour and the unpleasant odour. Oil and coolant mixing is one to take seriously, because the engine isn't being cooled or lubricated the way it should. On this engine the oil cooler is a heat exchanger that runs engine oil and coolant past each other so the coolant can carry heat away from the oil. When it fails internally, the two sides join up and the coolant contaminates the oil. The S5's engine is complex, so this needs experienced hands. The fix is to reseal or replace the cooler, then flush the engine, and one flush isn't enough, it takes several rounds to be sure no trace of water is left.
The diagnosis
The check confirmed coolant in the engine oil and traced it to a failed oil cooler, where the oil and coolant sides had joined, which is the contamination, the check engine light and the power loss. The head gasket and the block checked out, this was the cooler. And since the car had clocked over 100,000 km, it made prudent sense to replace the oil separator at the same time, while everything was open. That's an oil cooler reseal, a thorough multi-round flush of the engine, fresh oil and coolant, plus a new oil separator as preventive maintenance at the mileage.
The work
The oil cooler was sorted, the failed seals replaced with genuine Audi-spec parts and torqued to the manual figures, and the engine flushed, multiple rounds, until there was absolutely no trace of coolant left in the oil system. The oil separator was replaced with a genuine Audi-spec unit since it was over 100,000 km. The cooling system was drained, flushed and refilled with the correct Audi coolant, the engine refilled with the correct Audi-spec oil and a new filter, the system bled and pressure tested, and the oil pressure checked good. A road test confirmed full power back, a clean idle, the gauge steady, no check engine light, and the oil and coolant clean and separate.
The outcome
Full power back, a clean idle, the temperature gauge steady, no check engine light, the oil and coolant clean and separate, and the oil separator renewed for the long run. Sam got the S5 back running properly. Catching it early, resealing the cooler, and flushing the engine thoroughly meant the contamination was gone for good, and doing the oil separator while everything was open was the prudent call at the mileage.
