If one adds fresh water or new coolant or some additive to the radiator/coolant overflow reservoir, will it end up actively circulating through the system in a reasonably short amount of time (i.e. hours or days)?

While a mostly general question, my car is a 2007 Toyota 4-Runner, and reservoir of interest is the white thing right at radiator cap in center(ish) of pic:

enter image description here

(sorry about the inadvertent marketing...the best pic for me to clarify my Q had all of that embedded already.)

2 Answers 2


It depends on if you're actually talking about an overflow reservoir or an expansion tank. Expansion tanks are a lot more common in this day and age then they used to be, so this may be what you're considering.

If you are talking about an overflow reservoir, it will, over time, start to mix with the rest of the coolant. This take some time though (every car is going to be different). Usually, if you need to add coolant to this type of system, you should add it directly to the radiator. When done this way, it mixes the next time the engine is ran up to temperature.

If you are talking about an expansion tank, the new coolant starts mixing with the old immediately upon startup.

  • In looking around internet for an answer before posting, the terms seemed used interchangeably, so I didn't realize there was a distinction. I've updated the original post to include car info and a pic of object of interest. lt appears to mostly be referred to as a coolant overflow tank, though. (e.g. on Orielly and AutoZone and AdvanceAutoParts sites.)
    – AA040371
    Oct 3, 2023 at 0:56
  • 1
    @AA040371 - Yes, that is an overflow, not an expansion tank. I answered your question either way, so it should still cover what you're asking, correct? Oct 3, 2023 at 2:10

An expansion tank is a pressurised tank and will likely have multiple pipes connected to the boy of the tank. The tank is part of the circulatory system of the coolant system. The tank will usually have half of its volume containing air to allow for coolant expansion. If for some reason, the cooling system becomes over-pressurised, for example because too much coolant had been added, the pressure release cap will open and allow excess coolant/air to escape. It is usually just dumped on the road.

An overflow reservoir is not pressurised and also not part of the coolant circulatory system, it will likely have only one pipe connected to it. If the radiator pressure release cap opens due to excess pressure, the excess coolant is released in to the overflow reservoir. When the engine coolant cools down again and its volume shrinks, some of the coolant from the overflow reservoir will be pulled back into the cooling system to take up the space.

  • Thanks...I think I get it now and could identify one vs the other by inspection after reading your explanation...hooray! In 2nd para you used term "expansion tank" a second time...probably not intentionally?
    – AA040371
    Oct 3, 2023 at 12:34
  • Oops. All 3 occurrences of expansion tank in the 2nd paragraph should have said 'overflow reservoir'. Thanks.
    – HandyHowie
    Oct 3, 2023 at 12:39
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    An easy way to determine an expansion tank integrated into the cooling system; the radiator cap is missing, and a plastic cap is the pressure relief on the reservoir tank. The tank is plumbed into circulating coolant, under pressure, 100% of the time the engine's running. An overflow tank isn't pressurized and the radiator cap is on the radiator. When overheating occurs, hot coolant erupts and flows into the overflow tank. The radiator cap releases excess pressure and has a one way return valve; as coolant temps drop, a vacuum occurs to suck coolant from the overflow tank back to radiator.
    – F Dryer
    Oct 3, 2023 at 17:07
  • 1
    Easy rule of thumb is that most Japanese cars, like the OP has, still use overflow reservoirs while everybody else switched to expansion tanks decades ago.
    – user71659
    Oct 3, 2023 at 18:33

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