Either someone mixed coolants/and or the coolant needs to be changed. I would also recommend you get your cooling system professonaly cleaned out.
i used to own a 1997 mecury sable and the antifreeze also turned brown,it was very commen for a couple of years.the ford dealership said that the makers of the antifreeze made a defective product and they had to replace the antifreeze in many of their vehicles.they flushed the cooling system and put in fresh antifreeze and it never happened again.i went on to put 300 000 kms on it before i sold it. thats all that may be wrong with yours too.good luck!
this is common,get a bottle of prestone radiator cleaner pour in rad,run about 20 miles,let it cool down drain rad,flush it with valve open ,let idle use waterhose to flush,5 min will do it,close drain valve refill with collent.you may want to replace thermostat sence you will have to remove it anyway when you start the flush cycle.
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