Why has my coolant changed colour?

Hello Paul, my Mercedes C 200 engine coolant has developed a milky brown colour. My service bay mechanic suggests that it might be oil mixing with coolant. What is the cause and how can we fix it? Bosco

Hello Bosco, when your engine coolant mixes with oil, it turns to a brown milky colour. It is crucial to determine whether the engine coolant is mixing with engine oil or automatic transmission fluid (gearbox oil). 

Engine coolant is used to cool both oils. A sample of the Mercedes engine oil or gearbox oil can be looked at using a dipstick or draining a few millilitres. 
If traces of coolant contamination are detected in the form of a milky brown colour, that will help you determine which one of the oils is mixing with coolant. 
You need an experienced and well equipped mechanic to carry out this kind of inspection and repair. Should the traces of coolant contamination be found in the engine oil, a more likely cause will be failure of the engine oil cooler. 

This component has a section where engine coolant is channeled to help cool down engine oil by conduction. 
Overtime, delay to renew engine coolant makes it corrosive and this will perforate chambers in the oil cooler which separate engine oil and coolant hence causing a cross flow of oil and coolant. 
In this case, a close examination of the oil cooler will reveal the leaking point, replace it and service or renew both the engine oil (filter) and coolant fluid. 

On rare occasions, where a car has had overheating episodes, you may need to closely examine the cylinder head gasket in case it was damaged and allows cross flow of coolant and engine oil.
For Mercedes Benz vehicles with automatic transmissions, should you find traces of coolant in the transmission fluid, a more likely cause will be a damaged gearbox oil cooler which is incorporated in the radiator.

The delay to renew radiator coolant will cause corrosion damage of the separating plates causing intrusion of mix of gearbox oil in the engine coolant. 
Your mechanic will need to replace the radiator, flush the cooling system, clean the coolant reservoir before replenishing coolant and a gearbox service with a filter and automatic transmission oil (ATF) change.
What is coolant in your car for?
Antifreeze, or engine coolant, is a coloured liquid that is mixed with water to help regulate your engine during extreme temperatures. As the temperature outside changes from hot to cold coolant is pumped throughout the engine block to maintain an even operating temperature.