How to flush a radiator

How to flush a radiator

There are thousands fiery explosions every minute inside an internal combustion engine in a motor vehicle and combustion temperatures reach about two thousand degrees. All that heat has to go somewhere or the engine would only last a short time. The cooling system is what keeps the engine and its internal parts at a safe operating temperature.

About half of the heat produced goes out the exhaust system; the rest of it is absorbed by the engines block, crankshaft, pistons, rods, heads, and all of the other parts of an engine. If they are not cooled, they will quickly weaken and fail. The coolant, or antifreeze, in the cooling system transfers all that heat from the engine to the air. It is circulated by the water pump through the water jackets in the engine block, cylinder heads and back into the radiator where it is cooled by air flowing through and taking the heat with it. Over time deposits are collected contaminating the coolant and it should be replaced about every two years.


Step 1

Drain the cooling system

Place a drain pan under the radiator drain cock. Open the drain cock and remove the radiator cap to help it drain. If there is no drain cock or it is not accessible, put the drain pan under the radiator where the lower radiator hose connects. Loosen the hose clamp, give the hose a twist to free it up and then remove it from the radiator. Take care using this method as you will have a lot of coolant quickly draining from both the radiator and the the hose. Pour the old coolant into a container that can be sealed for safe transportation to your local recycling center.

Step 2

Coolant Reservoir

Remove the bolts or screws holding the reservoir to the vehicle. Remove the hose from the reservoir. Empty the coolant into your waste container.

Note: If the reservoir has a pressure cap, it is part of the cooling system and will not have to be removed as it will drain with the rest of the system.

Step 3

Fill with water and flush

Fill the system with water and a cooling system flush. Most radiators these days are aluminum. Make sure the flush you purchase is compatible with the radiator in the vehicle you are flushing. The people at your local auto parts store should be able to help you out.

Note: If the cooling system has been maintained and flushed according to the vehicle’s maintenance schedule, a chemical flush should not be necessary.

Step 4

Close the drain cock (or replace the radiator hose.) and replace the radiator cap. Turn the heater control in the vehicle to hot, so the heater core will be flushed too. Run the engine for the recommended time on the flush bottle, usually between five and ten minutes, then shut it down. The cooling system is under pressure when hot. Opening the system under pressure will cause scalding liquid to be sprayed on you. Let it cool off before you attempt to drain it again. When it is cool drain the water/flush mixture into the drain pan.

Step 5

Filling and bleeding

Look up the cooling system capacity in your owners manual. You will want to fill the cooling system with half coolant and half water, unless otherwise recommended by the owners manual. As you fill the system air gets trapped and tends to collect at the highest point. If your engine is equipped with a bleeder in the cooling system, open it up. If not, disconnect the highest hose, it could be the upper radiator hose or a heater hose.

Note: If the system pressure cap is at the highest point of the system, you should not have to remove a hose.

Step 6

Fill the system with the coolant/water mixture. If you have a hose disconnected fill it until you see it in the hose and reconnect it.

With the pressure cap off, run the engine until the thermostat opens. (You will see the coolant start flowing when the thermostat opens.)

Shut the engine off and let it cool. Add more mixture until the system if full. Some systems will have marks on the reservoir that say full and low, or full cold and full hot. Check your owners manual for the level recommendations.

Replace the pressure cap. Run the engine until it reaches operating temperature and check for leaks.

Things Needed
• Clean rags
• Drain pan
• Waste coolant container
• Screwdrivers
• Ratchet
• Sockets
• Coolant
• Cooling system flush (Optional)

Tips & Warnings
• Be sure that the engine is cold and the cooling system is not pressurized before removing the pressure cap.
• Safely dispose of waste coolant at a recycling facility.


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