The job of the coolant is to absorb the heat from the engine. Without it the engine would seize up.

It is the life blood of the cooling system, and its condition can tell you much about the causes of overheating if cooling problems develop.

Water based coolants
Water is very good at absorbing heat, but it has some drawbacks. Water freezes at 0°C. That's too high for some environments, so an antifreeze agent is required. This agent also slightly raises the boiling point, but not by much. Tap water contains a lot of impurities, so an agent is required to prevent them scaling up the system. Water is also corrosive, so it needs to have anti corrosive agents. Water is conductive, so more additives are needed to prevent the cooling system from acting like a battery cell, which will dissolve parts of the cooling system.

So we start off with drinkable tap water, and end up with a highly toxic chemical. Over time this protective coolant starts to turn acidic, which will begin to dissolve some of the metals in the cooling system. So it's important to change water based coolants every two years.

Which coolant to use.
Toyota have a coolant that has been specifically formulated for use in all their vehicles. It has the right additives in the correct concentration, and has been developed by Toyota's own chemists to fully protect their vehicles in all environments. Its premixed when you buy it, and must not be mixed with water. This means that you will need more of it, thus making it quite expensive. For a ten Litre system, the cost is around £35. There are many other brands of coolant that are mixed with water, and at a cost of £3.50 per Litre, on a ten Litre system with a mixture of 50/50 coolant/water, the cost works out at £17.5 A 30/70 coolant/water would cost only £10.50

Always follow the instructions on the bottle of antifreeze for your environment. Nearly all non Toyota coolants have to be mixed with water. Failure to do so may itself result in overheating,

Dave Bright, 25 Sep 2003.

Footnote from a Toyota dealer about the Toyota coolant...

There are actually 2 types of coolant for Toyotas, the normal RED coolant is for Japanese produced vehicles and there's also a GREEN variety for European built vehicles, quite what's the difference between the two, other than colour, I couldn't tell you, they are both the same price. Toyota recommend it to be changed every 3 years. It doesn't matter if a small amount of residual water is left in the system [after changing/flushing] it will mix ok, although you shouldn't add water simply because it will dilute the coolant and make it less effective (rust inhibiters etc).

Dave Coombs, 10 Oct 2003.