"Save this car" seems a bit concerning -- your only symptom is perceived high coolant level and some different colors in the reservoir. This does not necessarily mean a failure - sure, it could -- but without any further evidence I'd be reluctant to condemn anything. Hands on testing by a professional is highly recommend to give feedback as to what is happening.
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Hi, this is Bo, an auto expert on RepairPal's Car Genius team!
It sounds like your Mustang's cooling system is trying to tell you something. The excess coolant in the overflow reservoir and the contamination are definitely worth investigating. Since your oil looks clean and there's no overheating or leaks, the issue might not be too dire, but it's good to be cautious.
The brownish liquid could be rust or oil, which could indicate a couple of things. It might be a sign of a failing head gasket, but if your oil looks fine and there's no creamy residue, that’s a good sign. Another possibility is that there's some corrosion or sludge in your cooling system that's mixing with the coolant.
Replacing the radiator cap was a good move. Next, I'd recommend a coolant flush to get rid of the contamination and see if the problem persists. Keep an eye on the coolant and oil levels and check for any unusual behavior.
If the issue continues, it could be worth getting a professional inspection to rule out any larger problems like a head gasket leak or a failing radiator.
