Just joined. I wish to exchange the coolant on my Toyota Yaris as it had never been done in the 10 years I have owned the vehicle. I have bought the recommended, premixed coolant and 2 gallons of distilled water to flush the system.

I have looked at quite a few YouTube videos on changing coolant and there is something that I don't understand. First I will state the "problem" as I see it - then if someone can explain exactly what happens I will be grateful.

Is the following correct? When a water-cooled car or bike engine is cool, there are effectively two separate cooling reservoirs. One circulates through the radiator and is pumped by the water pump. The second reservoir surrounds the engine block. These two reservoirs are kept separate by the thermostat. As the engine heats up the water surrounding the engine heats up until it reaches a temperature when the thermostat opens and connects the two reservoirs. The (very) hot water from the engine block is pumped through the radiator to dissipate some of the engine heat.

Now, if I drain the coolant from the radiator through the drain plug at the bottom of the radiator when the engine is cold then am I only draining the radiator because the thermostat is closed? If I refill, surely I am only replacing the drained radiator coolant? The original coolant around the engine block will not have drained? I can't drain the radiator when the engine is hot because super-heated water will escape. If only the radiator has been drained when cold, surely the coolant around the engine block has remained? Then after refilling the radiator, when I run the car and it gets to operating temperature, the old (engine) and new (radiator) coolant will mix? Or if I try to flush with distilled water, at operating temperature the engine coolant will mix. An opposite problem will occur after I have flushed with distilled water, and start introducing coolant, the new coolant will be diluted by the distilled water around the engine block?

Can someone correct my (mis)understanding so I perform this coolant exchange correctly?

1 Answer 1


First of all, I think the idea of there being two reservoirs is conceptually correct, but technically wrong. While the thermostat does tend to divide the two, it doesn't completely separate the two. There is always coolant exchange between the two, but the thermostat keeps things moving in the right direction at the right time to keep the engine up to temperature. When you drain the fluid out of the radiator, some if not most of the fluid will drain out of the engine block as well. This happens because the thermostat isn't an all or nothing plug. As stated, it just directs fluid flow when the engine is running.

The main thing to remember about a coolant change (or basically any other fluid change) is, you'll never get 100% of the old coolant out of the system. You just do the best you can do and let the new fluid override the old. With the two gallons of distilled water will dilute whatever is left in the block as well as help you to flush it out when you drain it. After you put in the distilled water, you should run your engine up to temperature again so as to allow it to completely mix with whatever is left, then let the engine cool down. Once cool, drain it as completely as you can again. Then put the new coolant in. You could possibly do another round of flush of your system before you put new coolant in, but at some point you start seeing diminishing returns which really don't help anything in the long run.

  • Thanks makes sense and is reassuring. I have bought one of those no-spill coolant funnels (that are used a lot in the Youtube videos) to allow the air bubbles to "burb" without shooting coolant over the engine. I will probably flush at least 3 times with distilled water until the drained coolant runs fairly clear and similarly top up , drain, top up with coolant to try and get coolant of correct dilution throughout the system.
    – Clive Long
    Sep 12, 2023 at 17:25

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