The 2009-2011 Aveos have the engine oil cooler which, after about 4-5 years, starts slowly leaking oil into the coolant, and emulsifying into a mixture that dissolves rubber and plastics in the cooling system. Even after you've replaced the engine oil cooler, the damage has already started, and trace amounts of that sludge remains in the cooling system to continue to attack rubber and plastics. I've replaced 2 or 3 coolant overflow tanks, and I always keep a spare on hand in case it starts leaking. Although i replaced my engine oil cooler on my 2009 Aveo about 3 or 4 years ago, i know i have to continue to keep a close watch over possible leaking in all the hoses, coolant tank, and thermostat and water pump gaskets because it is near impossible to remove the trace amounts of sludge that dissolve plastic and rubber.

Deano