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Flush Coolant, Why, When, and How To Do It

Years ago people went to the mechanic yearly to test the coolant or perform a cooling system flush for winter to protect their cars from freezing. This was a good idea as most of the demands placed on the coolant system were the possibility of freezing, with a small focus on heat. In those days the engine coolant rarely exceeded 200 degrees Fahrenheit. The cars had extremely large cooling systems, radiators and multi blade mechanical fans that ran continuously.

The procedure then was to add some chemical cleaner to the radiator, allow it to run for 15 to 30 minutes, open the radiator drain, and insert a garden hose into the radiator to perform the coolant flushing. We performed this cooling system service typically every fall.

Today the cooling system runs hotter, typically around 240 degrees Fahrenheit. Cooling systems, radiators, and heater cores capacity is smaller. Full time operation mechanical fans have been replaced by on demand electric fans. This extra heat attacks the inhibitors in the engine coolant and required a rethink of the engine coolant formulation.

These inhibitors are what protects the metals inside the engine, radiator, heater core and water pump. An engine coolant replacement protects them from erosion and corrosion. Yes, you read that right, erosion. Manufacturers in their quest to obtain the government mandated fuel mileage requirements have lightened the weight of every component including cooling systems, radiators and heater cores. Heater cores are so thin that using worn out engine coolant can destroy a new one in just a few hours!

Measuring the inhibitor levels is impractical in the field so the next best thing is measuring the coolant/water concentration. This must be done by using a Refractometer coolant tester. The Refractometer uses a minimal amount of coolant that can be taken from the coolant reservoir, radiator or the engine block.

Inexpensive gravity float testers (floating balls) will not completely analyze the coolant concentration fully and should not be used. However this gravity float tester is still being used by shops and professionals that have not kept up with the changes.

The concentration levels should be between 50% and 65% coolant concentrate. This mixture will have a freeze point protection of -34 degrees Fahrenheit. If the concentration is below 50%, the cooling system must be flushed not simply drain or flush radiator and add. If the vehicle's coolant is low, drained out, or the customer has repeatedly added coolant or water to the cooling system, then the cooling system should be completely flushed.

What is the service interval?

Examination of manufacturers recommended coolant flushing interval is typically specified as 2 years or 30,000 miles for silicated coolants (typically green color) and 5 years or 100,000 miles for extended drain coolants like Dex-Cool (orange or gold in color). However the same Dex-Cool coolant used in the same vehicle operated in Canada, GM states“a service interval of 5 years or 240,000 km (150,000 mi)”.
Why the difference? Could it be the colder climate or is it strictly a marketing tactic? If you recall GM was sued many times for engine damages resulting from the recommended service interval. The reason for the dual statements is the first is partially based on laboratory testing and the later on real world experiences. Privately many manufacturers suggest 36 months or 50,000 miles as the coolant flushing interval for the extended life engine coolant.

Flushing procedures

Exercise great caution as the fluids are extremely hot and can do serious bodily harm to you! To avoid being burned, do not remove the radiator cap or surge tank cap while the engine is hot. The cooling system will release scalding fluid and steam under pressure if the radiator cap or surge tank cap is removed while the engine and radiator are still hot

Prior to the coolant flush procedure, the coolant reservoir (aka surge or overflow tank) must be removed, drained, cleaned and reinstalled before refilling the system. Also on a precautionary note bypass or remove the heater core from the coolant flushing by removing the heater hoses and inserting a bypass hose, this could save you the expense of having to replace it which requires a dash removal on almost all vehicles.

Don’t use one of the “Prestone flush kits” that has a “T” fitting for a garden hose as this can cause leaks and result in damages, also it can promote erosion of the metals in the cooling system during the coolant flush procedure from excessive pressure and flow. Drain the coolant and dispose of it properly. Refill the system using clear, drinkable water and run the vehicle until the thermostat opens. Repeat this process three (3) times to totally remove the old coolant or until the drained coolant is almost clear.

Once the cooling system is completely flushed, mix a 50%-60% concentration with the correct engine coolant concentrate and gently pour it through the heater core until fresh engine coolant emerges. Then reattach the heater core into the cooling system.

Refill the cooling system to a 50%-60% concentration with the correct engine coolant concentrate. Using the wrong engine coolant may cause premature engine, heater core or radiator corrosion.

Then slowly add clear, drinkable water to the system until the level of the coolant mixture has reached the base of the radiator neck or the level indicator on the surge tank. Wait a few minutes and reverify the coolant level. If necessary, add clean water to restore the coolant to the appropriate level.

Once the system is refilled, reverify the coolant concentration using a Refractometer coolant tester. The concentration levels should be between 50% and 65%. If you use an improper coolant mixture, your engine could overheat and be badly damaged. Then add the appropriate stop leak pellets to the coolant as specified by the manufacturer to correct micro porosity in the gaskets.

For The Professional

Verify that no electrolysis is present in the cooling system. This electrolysis test can be performed before or after the system has been repaired. However it must be done and if any problems are found they do need to be corrected to maintain the life of the engine coolant and not have a recheck.

Use a digital voltmeter set to 12 volts DC. Attach one test lead to the negative battery post and insert the other test lead into the radiator coolant, making sure the lead does not touch the filler neck or core. Any voltage reading over 0.3 volts (3/10 Vdc) indicates that stray current is finding its way into the coolant.

If you don't own a good Digital Multi Meter (DMM) you can get one HERE

Electrolysis is often an intermittent condition that occurs when a device or accessory that is mounted to the radiator is energized. This type of current could be caused from a poorly grounded cooling fan or some other accessory and can be verified by watching the volt meter and turning on and off various accessories or engage the starter motor.

If your shop has a cooling system flush and fill machine use it and follow the tool manufacturers recommendations for proper operation. If it is not available, follow the procedure outlined above.

Do not mix the orange and green colored engine coolant when adding coolant to the system or when servicing the vehicle's cooling system. Mixing the orange and green colored engine coolants will produce a brown coolant which may be a customer satisfaction issue as it looks like rust is present.

Remember to add the cooling system “stop leak pellets” that the manufacturers sell. I personally like the GM pellets because one package can typically service two vehicles, most GM dealers have them in stock, they do not discolor the engine coolant like “Bars Leak”, they are designed to stay suspended in the engine coolant and do not stop up radiator or heater core tubes, and they work extremely well especially on the “Northstar” engine.  

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