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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.

Narcotixs
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6 Answers6

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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?

user33191
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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.

Nick C
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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

user33209
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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.

Community
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DaveInAZ
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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.

Zoomzoom
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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

RRH
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