I have a new (to me) car that it appears hasn't had a coolant fluid change ever, now with 120k miles. It is an 04 Corolla.

I am planning on flushing the system with distilled water a few times. But it seems to me that if I add a 50/50 mix of fluid back after the last drain, all the water left in the heater core will dilute the 50/50 mixture even more.

So since they say about half the liquid remains in the heater core after a drain, that I should add undiluted pure antifreeze to the system. That way it will mix with roughly half the water in the engine with the pure coolant I put in.

Does this sound correct? Am I right to add 100% coolant to my system after a 3 or 4 flushes with distilled water? Which would leave me with about a 50/50 mix (after driving a while and everything mixes).

  • 1
    Something that one one on here is mentioning, coolant isn't necessarily good. The more coolant you add the less heat the mix can hold and therefore take to the radiator. If you live in a hot location, you actually want more water to coolant. Graph of heat capacity google.com/… And the boiling point counter argument mtfca.com/discus/messages/331880/398498.gif May 14, 2016 at 15:29

4 Answers 4


Yes what you presume is correct. Depending on total volume of coolant (different for each vehicle) how much pure AF I put in, anywhere from 1/2 to 1 gallon, then top off with 50/50 mix.

It is better to be over the 50% mix than under, you can go as high as 70%, so don't worry about putting a little too much pure AF after flushing the cooling system with water.

  • 1
    I always mix it a bit high so I can add straight water when it leaks.
    – rpmerf
    May 13, 2016 at 19:25
  • 1
    I like mine a bit high as well, for exactly the same reasons. Water is easy, antifreeze may not be ... May 14, 2016 at 0:44
  • In this video, the popular chris fix does a distilled water flush and then adds 50/50 coolant so no pure antifreeze. Is he wrong, might this cause a problem considering he added a 50/50 mix and not pure antifreeze? May 13, 2018 at 18:58
  • After a flush you still have some pure water left behind in the engine block and other places, so mixing it beyond 50-50 is recommended to keep a total near 50-50 or higher. Unless you can completely drain the bock and heater core of pure water which is difficult to do. pure water you should add more than a 50-50 mix.
    – Moab
    May 13, 2018 at 19:03

The coolant capacity of the 2004 TOYOTA COROLLA 1.8L 4-cyl Engine Code [R] 1ZZ-FE is 6.9 quarts. To accurately ensure you have a 50/50 mix in your system after a flush (assuming you actually flush it until you have clear liquid draining out), is to add 1/2 of the coolant as straight coolant (not 50/50 mix), then fill the rest as distilled water.

For instance, let's say you need to fill/run/drain the vehicle 4 times to get to where the coolant is clean on the last go around. Yet, when you drain the cooling system, you only get 5.5 quarts out. That means you have 1.4 quarts of water left in the system. You'd then proceed to add 3.45 quarts (let's just call it 3.5 quarts to make it easy) into the radiator and then fill the rest with distilled water. At first you will be just a bit high on antifreeze, but a 50/50 mix is optimal for the system. Having just a touch more is not a bad thing. As you burp the system, it will get closer to a 50/50 mix overall because you'll be adding more water. When you've completely burped the system, then mix what antifreeze you have left with distilled water to make a 50/50 mix external of your system and fill the overflow reservoir.

After you have added everything back in and it is well mixed, you can utilize the hydrometer to ensure what you've done is correct, but I'll assure you, if you follow these simple steps, your system will be spot on.


You can use a coolant mixture tester to test the coolant to water ratio. Any auto parts store should carry them.

Prestone Antifreeze/coolant tester

  • My car (2013 Scion tC) does not have a radiator cap, and while the engine draws from the resevoir when it needs additional coolant, I don't believe it ever returns coolant to the resevoir. Therefore, I wouldn't be able to use this to check the ratio of coolant actually circulating in my engine. Jul 29, 2018 at 21:30

This answer is very helpful! After reading the factory manual of VW Tiguan, I also realized that it is the freezing temperature of the mix rather than the concentration itself is the key factor in flushing. I decided to but a refractometer for my flushing job just to make sure the vehicle is ready for some winter trip!


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