Q: under the hood/ AC on 2002 Buick Century

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I recently purchased a used car, and while doing my own inspection, I noticed that there was an oil like substance in my coolant reservoir. In doing some research I noticed some people talking about mixing the wrong types of antifreeze can turn the color brown. My question for you is what exactly is this in my coolant reservoir, and how do I fix it?
(1) Answer
You could have transmission fluid in your coolant from a transmission cooler leak. You could have engine oil in your coolant, from an internal engine issue. It's too hard to know until you can isolate the situation. Is there any coolant in either your transmission fluid or engine oil? Check those fluids. A flush of the cooling system may be a great way to start to "control" the situation, and look for any further oil "variables" in your fresh coolant. Good luck.
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