There is more to changing your coolant than just draining it. You need to remove all the fluid and simply draining it will only remove half of the old coolant mixture and leave any deposits of rust and debris. Coolant travels through rubber hoses through the water pump, radiator, heater hoses, heater core, aswell as the engine block and head. All these systems need to be flushed clear of old fluid and debris.
Caution: Antifreeze is toxic and must be disposed of properly at a fluid-recycling center.
Keep it out of the reach of pets and children, and avoid skin contact. Avoid spilling it on painted finishes as it attacks paintwork.
Note: Never remove the radiator cap or any hoses unless the engine is cool. Doing so can result in serious injury.
Cooling-system back-flush kit (includes fittings and clamps)
Nut-driver or socket wrench
Make sure that teh engine is cool and remove the radiator cap to relieve any pressure in the system
Located at the bottom of the radiator there should be a small tap or stopcock. Open this and drain fluid into a catch basin (It is a good idea to keep an eye on the level of fluid in your catch basin so that it doesn't overflow). If your radiator does not have a stopcock then an alternative method is to remove the bottom radiator hose.
The easiest way to flush the system is to install a back-flush kit. Follow the kit's installation instructions.You can repeatedly fill and empty the system to aid flushing if you do not have a kit but this method is not as efficient at removing debris. Remember to turn off radiator tap before refilling.
Once you have finished flushing the sytem drain all water left in system and add the mixture of antifreeze and water after closing the drain tap. A proper mix should be 50/50 and should be added until full.
At this point the cooling system is probably only half full because of the air that still remains in pockets. this air has to be removed. This is called bleeding.
Make sure that the car is level.
Some cars have a bleed valve located at the highest water point on the engine. If not leave the radiator cap off and start the engine. Top up the fluid level as it recedes and keep going until there are no more bubbles of air. This should take a few minutes. When you are happy that there is no more air left in the system then replace the radiator cap and take the vehicle for a drive watching that the temperature of the car DOES go up but does not go above the normal level. I the temperature does not seem to be rising then there still might be air in the system (An air pocket that the sensor is sitting in and therefore not feeling the temperature of the water.) Repeat the air bleeding process until effective. Check the level of the fluid the next day to make sure that it has not receded.