Yellow residue in coolant - E90

[upwindskipper]

I have ignored a minor coolant leak for a few months, waiting for warmer weather to tear into it. Went to top it off and found yellow scum in the overflow tank. I have used only BMW coolant. I did the transmission fluid/pan change about 20,000 miles ago. I'm familiar with the tan-to-brown engine oil and water mix but this is yellow. I'm suspecting the trans cooler and only hope that I have not compromised the transmission. I have drawn some engine oil and sent it out to the lab, will be a week (with no car) waiting for results. The sample was a nice brown, no milkiness.

When BMW transfluid and BMW blue coolant mix, is yellow the result?

[catalinirimia]

When transmission oil gets mixed with antifreeze the color would be a light yellow, white-ish... and foamed. Im not sure of the pressure în the oil side of the cooler but it should be anyway higher than the coolant pressure. So if that's the case your transmission will not get damaged by coolant inside of it, so you can rest assured that There will be no trouble There.
You will need to locate the leak and make sure that you clean thouroghly the cooling system afterwards

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