From your description it doesn't seem like oil. As for why it continues to look brown, you probably haven't flushed it enough times. When I had an oil-in-coolant condition on one of my vehicles, it took the mechanics at the dealership 12 flushes for the coolant to come out clear. And this was after having replaced most of the cooling system's hardware (...


Air is getting into the cooling system to cause rust. The most likely place is the radiator cap ( not the reservoir cap ). The radiator should be "sealed" , the only exchange should be into and out of the reservoir which acts as an air lock to keep air out.

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