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I drive a 2016 Scion FRS, and based on the maintenance schedule of the coolant:

1st replacement interval is 11 years/137,500 miles (220,000 km). 2nd replacement interval is 6 years/ 75,000 miles (120,000 km) after the 1st.

...I don't really understand the point of replacing the coolant. Why would it need to get replaced if you're constantly replenishing it? Especially during a long period of 11 straight years, wouldn't it have been replaced several times over?

Edit #1

I said "constantly replenishing". Constant could be any frequency. The definition of constant means occurring continuously over a period of time. So let me be specific, "topping off when needed". Also my car is still relatively brand new, under 10k miles. There aren't any signs of leaks that I know of, so it's doubtful I have a leak.

I already answered my own question prior to looking at any of the answers here, but correct me if I am wrong. Basically, by "topping off", I am diluting the ratio of impurities to non-impurities in the coolant solution, but I am never removing the impurities themselves. When coolant gets used, it evaporates, leaving the impurities behind, so replacing the entire supply of coolant is necessary as a method to carry away the impurities. The only person who came close to this answer is user33191.

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    You should not need to "top off" coolant. It is a 100% sealed system with safety and overflow vents -- neither should be used in normal operation. There is still unusual behavior with your car (or you as an owner) if you are adding more water to your cooling system outside of the service flush intervals.
    – Bryan Boettcher
    Oct 24, 2017 at 15:54

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The logic behind replacing the coolant, rather than just topping it off, is to remove impurities.

If the reason you have to add to it is that there is a slow leak somewhere, you may get by with just adding some now and then. But if the nature of the problem is that that coolant itself is being depleted, without removing any impurities, then yes, it has to be replaced.

What does it look like when you drain it off? Does it look like fresh coolant?

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    Welcome to the site. This is a good answer. Just to give a pointer; please avoid asking clarifying questions within an answer.
    – CharlieRB
    Oct 23, 2017 at 12:22
  • Even if the OP is losing coolant through a slow leak he probably still would benefit from flushing the system at the scheduled intervals to remove any sediment/sludge/debris/etc that are building up in non-leaking low points in the system.
    – Dan Is Fiddling By Firelight
    Oct 23, 2017 at 15:00
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    @CharlieRB, I don't think that's a clarifying question. It seems like a guiding question to help the reader judge how the previous paragraph applied to his/her particular situation. The following logic seemed pretty clearly implicit to me: If it looks like fresh coolant -> maybe you just have a slow leak and you can top off without replacing; if it looks sludgy -> you need to replace it.
    – Wildcard
    Oct 24, 2017 at 3:34
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    @Wildcard Agreed. It looks like a rhetorical question to me.
    – JBentley
    Oct 24, 2017 at 7:33
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    Hmm...I can see your point, so thanks for the feedback. It is not clear though. Maybe we should let the OP clarify what the intent was. It is not a big deal. Was just trying to help the OP improve the answer.
    – CharlieRB
    Oct 24, 2017 at 11:32
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You shouldn't be constantly replenishing the coolant - if you are, there's probably something wrong with your car!

Modern car cooling systems are designed to be almost maintenance free, so you should only need to drain and replace it after a long interval (e.g. the 11 years you quote), rather than every two years with older systems - obviously you should still check it regularly, but the coolant lasts much longer than it used to.

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    While true this doesn't really answer the question which is at its core "why replace not just top up (if required)"
    – Richard Tingle
    Oct 23, 2017 at 21:12
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    I disagree, the question specifically mentions that continual replenishment over time will have replaced the coolant many times before the change interval is reached. That means OP expects gradual coolant loss as a normal thing, which it is not in modern cars.
    – barbecue
    Oct 25, 2017 at 0:10
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If you have a constant need for coolant for replenishment you either have a problem leaking coolant or your engine is running entirely too hot. Which ever the problem may be, you have to keep the coolant level up in your car. If you don't your engine will overheat and you will be looking at damage 10 fold the price and hassle it would take to fix the coolant leak problem

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Excerpted from here;

Generally, coolant degradation is accounted for in manufacturers’ “recommended use” intervals. Conventional coolants containing silicates degrade primarily due to rapid inhibitor depletion. This is because silicates lay down protective layers over the system components as part of their protection mechanism. Therefore, coolant inhibitors must be replenished or changed regularly to ensure the surfaces will remain protected if the silicate layer is disturbed.

In general, coolants degrade over time as the ethylene glycol breaks down into primarily glycolic and formic acids. Degradation occurs more quickly in engines operating at higher temperatures or those that allow more air into cooling systems. The coolant should be tested on an annual basis if it is intended to operate the system for several years between coolant changes, and particularly where the coolant is used in severe applications. One test ensures the pH is still above 7.0. Some coolant technologies can protect as low as pH 6.5, however, it is typically not good practice to allow a coolant to operate below a pH of 7.0. Glycol breakdown products are acidic and contribute to a drop in pH. Once a coolant has degraded, due to glycol breakdown and pH drop, engine metals are at risk for corrosion. Coolant degradation can be slowed by using coolants with extended life inhibitors and by ensuring that the equipment is operating correctly and within designated design limits.

Testing for corrosion inhibitors is another method of checking the coolant condition. While extended life inhibitors do not typically need to be tested as long as proper usage recommendations and correct fluids are used for top-off, conventional inhibitors deplete and need to be tested. Other than tests for nitire and molybdate, most conventional coolants require either continual supplemental coolant additions (SCAs) or lab analysis to ensure proper performance.

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  • I like this as it covers that coolant it self degrades over time/user not just contaminant build up.
    – xQbert
    Oct 24, 2017 at 16:01
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I think folks who answered so far may have missed the fact that OP's question is based on the official manual from Toyota/Scion. OP is not saying there's something wrong with his car.

I'm not a mechanic, but based on my limited knowledge, my best guess is that it is a way to make sure the cooling system actually get inspected thoroughly after 11 years. Afterwards, since the car would be fairly old, the shorter interval of 6 years makes sense because leaks are more likely to happen at this point.

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    He says "if you are constantly replenishing it", but you shouldn't be. In a modern car, you should almost never have to add coolant, unless something goes wrong. We're reading that as "I have to add coolant all the time, so I'm already replacing the coolant, right?"
    – JPhi1618
    Oct 23, 2017 at 18:00
  • I don't understand your logic -- requiring that the coolant be inspected doesn't require that it be replaced. If the goal was to ensure periodic cooling system inspection, the manufacturer would call for inspection. And indeed, the 2016 Scion FRS does have a coolant inspection interval of 30,000 miles.
    – Johnny
    Oct 23, 2017 at 21:18
  • What's called 'anti freeze' in coolant in fact also contains other chemicals to prevent corrosion inside the system. Those chemicals do their job being used up in the process. Hence, like an oil change, they should be replaced after some time.
    – JimmyB
    Oct 24, 2017 at 12:10
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I think the first reason for replacing the coolant is that the anti-freeze properties of the liquid are deteriorating over time. When you leave your car outside in -30 C then the 10 year old liquid may turn solid. The second factor is probably the corrosion inhibiting. Being not competent in chemistry, I can just guess that large organic molecules in the antifreeze are breaking up due to the high temperature. And no, there is normally no need to add coolant when there are no leaks in the system

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    How do you claim that anti-freeze properties deteriorate over time? That would essentially mean that ethylene glycol turns into something else in a chemical reaction. I haven't heard of any such chemical reaction.
    – juhist
    Oct 23, 2017 at 17:39
  • @juhist: That's a good question, but lots of modern vehicles use awful coolants that are not ethylene glycol (i.e. dexcool etc.) and that react in messy ways with contaminants.
    – R.. GitHub STOP HELPING ICE
    Oct 24, 2017 at 18:54

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