Why is there foam in my overflow bottle

If you are speaking of the cooling system - it's hard to know -- until you check into what the contents of the bottle are. Could there be some coolant contamination? Perhaps. But I wouldn't jump to conclusions until you possibly flush the cooling system and replace the coolant - making sure to purge all air out of the cooling system (air can also cause foam...). Use this link to find some shops near you that can help with your concern: www.repairpal.com/repair-shops