Flush the water through until it runs out clear.
Over a period of years, sediment builds up in a car’s cooling system even if antifreeze containing corrosion inhibitors is left in all year round.
Eventually the sediment will start to obstruct the water passages in the radiator and engine. Such obstruction causes overheating of the engine which, if it becomes severe, can lead to engine seizure.
If your engine is overheating and you have eliminated other possible causes, such as leaks (See Adjusting the static timing), the water passages are probably choked or narrowed by sediment. To clear them, thoroughly flush the radiator and engine.
Check whether there is any risk of wetting electrical components, including an electric cooling fan. Protect such components by covering them with a plastic bag. It is vital to do this before rinsing dirt from the outside of the radiator. Use plastic bags to cover the nearby electrical components.
Before flushing, drain the old coolant from the system and discard it the sediment deposits will have contaminated it. Remember, however, that the antifreeze contained in the coolant is poisonous. Have a large container ready to collect it, for disposal later at your nearest dump for poisonous wastes.
With the engine cold, remove the pressure cap from the radiator or expansion tank. Turn the heater control to hot.
If the radiator has a drain plug, remove it; if there is a drain tap, open it – look in your car handbook to see which way it turns, as it can be broken easily.
If no coolant runs out, gently poke the hole clean with a piece of wire, or unscrew the tap completely.
If there is no tap or drain plug, disconnect the bottom hose at the radiator end.
There may also be a drain tap on the engine block. If so, open it.
How to flush a car radiator
Fully loosen the hose clips and pull them back along the hose. Pull off the hoses taking care not to strain the metal stub pipes.
Disconnect both hoses from the radiator. Push the end of a garden hose into the top radiator connection stub, and seal it with rags.
Refit the radiator pressure cap and turn the water on full. Flush until the water runs clear.
Check by catching some water in a glass jar and looking for sediment which you can see only in still water.
Make thehosepipe a tightfit by jamming it inwith rags.
Flush the water through until it runs out clear.
If the water does not clear in a few minutes, reverse flush the radiator. Seal the hose into the bottom stub. Tie a long plastic bag with a hole in the bottom to the top stub, to direct the coolant away from the engine and electrical components. Flush until the water runs clear.
Reverse flushing a radiator
Turn the removed radiator upside-down or stand it on end so that you can flush it in the reverse direction to the normal flow. Tilt if necessary to bring the outlet to the lowest point.
If simple reverse flusing does not clear the radiator, take it out of the car.
Prop it upside down and seal the garden hose into the bottom stub, which is now at the top. Flush hard until the water runs out clear.
Flushing the engine block
Disconnect the top hose from its stub on the thermostat housing above the water pump.
Remove the thermostat before flushing, noting how it fits.
Unbolt the top of the housing and lift out the thermostat, noting which way up it fits. Refix the top of the empty housing.
Seal a garden hose into the housing stub. Flush until water runs clear from the bottom hose. This is reverse flushing: water normally flows up the engine.
Refit the radiator if you removed it, and put back the thermostat. Fit a new gasket in the thermostat housing, after smearing both sides of it with non-setting sealant. Bolt back the top.
Reconnect the hoses, renewing any damaged hoses or clips or any that are more than four years old.
Refill the system with clean water. Run the engine up to its normal working temperature and check for leaks.
Add antifreeze (See Checking and topping up car antifreeze coolant) only when you are satisfied that there are no leaks.
Even in summer, use an antifreeze mixture — it contains anticorrosion additives. Ordinary tap water can cause rapid corrosion and partial blocking of the cooling system.
The coolant mixture is usually good for two or three years’ winter and summer use, after which it should be replaced.
Cleaning radiator fins
Hose water through the radiator fins from the back of the radiator to the front, to clean them thoroughly.
Brush loose dirt from the fins of the radiator core with a nylon brush – not a wire brush, as the soft metal core is easily damaged. Do not poke it with a wire or a screwdriver.
If the core looks oily, spray it with a proprietary oil dispersant or apply an oil solvent with a soft brush.
Protect the electrics, then hose water through the fins from back to front.
