dirty coolant

Not that I'm the expert Jono, but the coolant breaks down chemically and becomes acidic that eats into your alloy bits of the engine and radiator. Generally, the white rubbish that coats your thermostat and inside of your radiator is rubbish thats been eroded away.

I try and change mine annually, but often that goes our to every 2 years or so.
chris

Depending on your engines and cooling systems age, material of makeup etc... Also if your cars been boiling/getting hot etc..

Most of the discolouration is rust!

But I change it every year, its only ~$20!

Gday Jono
Dont quote me on this becauce im no expert either but some where deepin the vault i seem to remember being told that it is like a ware indercator that when it goes dark it is a sign that it is time to change it how true that is i dont know but from experance i can tellyou if you run an ali head make sure you use not just coolant but the correct coolant for your motor , i made the mistake of using water in stead think i would save money it stuffed the hesd in no time flat very expensive mistake,thats when i found that there are diferent coolants for different motors . hope this is of some help but dont take the bit about dis colouration as gospel as my brain is full of tit bits some true others not so sure on
Regards Bruce

I would say that cooling system servicing is almost as important as changing your oil.
And it is very important you use the correct coolant.
I use Natra Coolant it is expensive $13.50 a litre and need 50% mix for a diesel but get 3 years out of it and I test it all the time with some castrol test stips that are about $7.00 for container.

Have a 12 year old GQ Patrol Diesel (no alloy head etc.) and have changed the coolant ATLEAST twice! (If that!)
Have never had a cooling or corrosion problem and it's clean in the radiator etc.