How to flush a radiator

How to flush a radiator

A car’s radiator is the heart of its cooling system. To operate properly it must be clean and clear of blockages and there must be no leaks in the system.

The radiator’s function is to get rid of the heat from the hot water circulating in the cooling system. The hot water enters the top tank in the radiator, filters down through the core fins of the radiator and cools, then returns to the engine through the water pump.

If you have a factory sealed radiator there is no need to do anything unless you detect leaks.

If you have a non-sealed type of radiator you need to check the water level at least once a week and top it up if necessary.

The non-sealed radiator should be checked regularly and cleared of leaves, insects, paper, or any other obstructions which can restrict airflow going through it. Do not scrape objects off as this may damage the radiator core. Use a hose to soak and wash things off.

Ideally a radiator and cooling system should be flushed out every six months. This helps avoid blockages in the core and accumulation of rust in the system.

Too often motorists neglect radiators until something bad happens which is why radiator problems are perhaps the most common cause of on-road breakdowns.

How To Flush and Clean a Non-Sealed System

  • Park the vehicle on a level surface with the engine switched off and heater switch turned to on position. Cover the engine to protect it from splashes.
  • Take off the radiator cap. Turn tap at bottom of the radiator to let water out and disconnect upper and lower radiator hoses.
  • Inspect the condition of the hoses. If they are brittle or soggy replace them with new ones.
  • Inspect the old liquid that comes out of the radiator. If it is clean, that’s good. If it’s dirty, especially if it’s brown with rust, the system needs extra special flushing. Whatever happens, flush the system anyway.
  • Put the water hose into the top of the radiator and let it gently flush through for a while, hopefully until the water runs clean again.
  • Put the hose in the bottom radiator outlet and reverse flush. Then flush from the top again.
  • Flush through the engine cooling jacket to remove any deposits.
  • With a very dirty radiator use a can of radiator flush. This involves putting the hoses back on, topping up with water again, inserting the flush treatment, and running the engine for a while to loosen corrosion deposits. Then you empty the radiator again and flush again with a water hose, as previously described.
  • When flushing is completed, close the radiator tap, refit hoses and fill the radiator with coolant. Most modern coolants include corrosion inhibitors and are preferable to plain water.
  • Before replacing the radiator cap, check its condition. If it is old or rusted, put a new one on.
  • Start the engine and let it run while checking for leaks.
  • Go for a drive and check that the cooling system is functioning properly. After this, keep the engine running and again check for leaks.
  • When the engine has cooled down check the coolant level to see if it needs further topping up.


If your radiator has a leak you can purchase a can of leak sealer. This is put in your coolant but is only for small leaks and is only a temporary measure. You can’t rely on this in the long term.

If a radiator leaks it’s best to take it to a specialist for repair or get an exchange unit.

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