Sunday, July 14, 2024 Detailed Auto Topics
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Should I replace my own coolant or should I have it replaced?

Part I covered the reasons why and when coolant should be replaced.  Replacing coolant is not difficult, but it must be done correctly. Improper replacement of coolant is worse than not changing at all. Many shops do the job improperly. Using a new car dealership is also no assurance the job will be done properly. Best is to find a cooling system specialist that uses OEM coolant and distilled water, or do the job yourself.

Properly replacing coolant requires a bit of understanding and some information. Service data for the vehicle will tell, the proper type coolant, the system capacity and the location of engine-block drain plugs. We also need a method of catching the old coolant, measuring equipment and a clean place to work.

Why we must use distilled water

always use distilled water in a cooling system

City water often contains minerals, chlorine, fluoride and other contaminants. It is NOT suitable for a cooling system. Always ask a service shop it they use distilled water. The answer will tell if this is a good shop to do the job. If they say yes, ask to see a distillation machine or bottled distilled water on hand.

Several vehicle makers have switched to premixed coolant, to avoid this problem. Pre-mixed OEM coolant is great, but very expensive as it take twice as much to do the job. On vehicles where premix is the original coolant we must use it. When the original is a concentrate, we must properly mix it before it is added to the system. Many shops fail to observe this necessary step and cause major problems as a result.

Pre-mixing coolant

Mixing coolant is a very precise science to prevent disaster

Unless they specify premixed-coolant, proper replacement requires a method to measure the mixture accurately. Concentrate should be pre-mixed with the distilled water, BEFORE it is added to the engine. Coolant is much heavier than water and will NOT always mix in the engine block.

With many engines, coolant flows only through the cylinder heads. The lower block fills with liquid but does not flow. Concentrated coolant can enter this area and remain there, because of its weight. Water with a smaller amount of coolant will circulate through the upper part of such a system. This promotes corrosion as the mixture is inadequate to protect the metal in the system.  Play it safe by mixing the coolant and distilled water, before pouring them into the engine.

Precise measurement is necessary to achieve a proper 50/50 mix. Premix the coolant with an equal portion of distilled water. For instance, mix one gallon of coolant with one gallon of distilled water. We stir or agitate this mixture to blend thoroughly, before pouring it into the vehicle. This step cannot be over emphasized. Failure to measure properly and premix coolant is a leading cause of cooling system problems today.

Removing the old coolant

Removing as much of the old coolant as possible is important. Draining the radiator will remove a little more than half. We replace only about 60% of the coolant, leaving 40% depleted. Removing the engine drain plugs does a better job. This is no small task on many engines and some do not have drain plugs.

Coolant drain plug on a 4.8 and 5.3L GM V8 engine

Shown above is the 5.3L GM engine that is very popular. We need A large allen type socket to unscrew this plug. With the vehicle on a lift, the job is easier. The 5.7L and 4.3L GM engines have smaller plugs, midway of the engine block and lower. Many also have a knock sensor on the right side. Removing these are much easier.

Badly corroded freeze plug shows cooling system problems

In cases where no plug is present, we may remove a freeze plug. This has the added advantage of allowing us to see the condition inside the cooling system. A new plug must be available to replace the old plug and this is sometimes difficult. When severe corrosion is found, find a professional and have them treat the system.

Before draining, check service-data for the cooling system capacity. We use this to tell us the percentage removed. Catch and measure the old coolant and compare this with the capacity of the system. For instance, if three gallons comes out, from a four-gallon system, we have removed 75%.

When removing the old coolant is not possible

When removing at least 80% of the old coolant is not possible, we can repeat the procedure. Drain as much as possible and refill the system with coolant and distilled water. Next, with the heater on high, run the engine until it is normal temperature. Let the system cool and drain it again. This should get a much larger percentage of the depleted coolant out. Never use water for this purpose as it will only dilute the remaining coolant. This further reduces corrosion protection.

Flushing the cooling system

Flushing a system is not necessary and may introduce harmful chemicals. Never add anything to the cooling system, other than coolant and distilled water that has been pre mixed. Doing a complete drain and refill is far better than attempting to flush a system. Servicing the system, before we deplete the old coolant, will help prevent a rust build-up. Otherwise, we will need a multiple drain and fill, using coolant and distilled water.

Flushing with water will severely dilute any coolant that remains. Water remaining in the engine will reduce corrosion protection and produce an unknown mixture. If the system is badly rusted, a professional cleaning, removing the freeze plugs, may be the only solution.

Purging air from the system

Reservoir system works as a syphon 

When we drain coolant, the radiator and engine fill with air. Older vehicles had the radiator situated higher than the engine. This allows air to flow out as coolant is added. Some vehicles have engines mounted higher that the fill point for the system. Such engines may have a flowing surge tank or a standard reservoir, as shown above.  Many will also have bleeder screws to remove trapped air. 

Coolant funnel makes bleeding the system easier

On engines with the bleeder screws bleeding any air trapped in the system is important. Service-data lists many procedures for different vehicles. We will need such information to do the job. Bleeding a system is far easier with a special funnel made for the purpose. These devices attach to the fill point and raise the coolant level. Gravity forces the coolant down and the air out, when opening the bleeder-screws. Consider investing in such a tool, when changing coolant. It makes the job far easier.

Surge tank system flows and tends to bleed itself 

Many modern systems have switched to a flowing surge tank, rather than the old syphon-type reservoir. This is a far superior system and air will be forced out with the self-bleeding design. To bleed such a system, after filling, put the heater on high and allow the engine to come to normal operating temperature. Let it cool fully and top off to bring the level to the full mark. The funnel mentioned above is handy in this process. Rechecking the coolant level a few times, over the next few days is also best. Any trapped air will cycle out, but will result in the coolant level dropping.

Don’t forget your cap

A radiator cap must seal pressure and prevent air from entering the system

Another very wise precaution is to replace the radiator cap. Caps seal the system, provide pressure to prevent boiling and keep air out. A worn cap may not leak, but can fail to seal air out. As the system cools, the coolant contracts. This may draw air past a worn cap. Any air entering the system will greatly speed the rate of corrosion. A new OEM radiator cap is cheap insurance. Aftermarket caps do not always fit properly and may be far lower in quality, than the original.

Proper cooling system maintenance, before we deplete the coolant, will prevent most corrosion. This can save a huge amount and help your vehicle keep its cool.

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