Coolant (antifreeze) contains more chemicals than just ethylene glycol and the corrosion inhibitors are consumed over time. The new universal coolants have better corrosion inhibitors now and are good for 5 years unlike the old green coolant that was only good for 2 years. Use distilled water and mix it 50/50 with the coolant.
I would also do a citric or oxalic acid flush to remove any rust and scale. See
Cooling System.
