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How long to let the coolant circulate before testing it with a hydrometer?

I flushed out my cooling system through the radiator with distilled water four times. It’s a 3 gallon system, but I could only drain/fill a bit more than a gallon each time.

I figured that by the end of it, it’s mostly distilled water so I only added antifreeze concentrate at the end. How long should I let the system circulate before I considered it well mixed and ready to test the concentration?

Until the thermostat opens…


It may take a little idling after the stat opens to equilibrate the sol’n. But not much. Make sure to turn on the heater to max, so the coolant goes fully through that path too. Presuming it is summer and warm where you live, you have some leeway. Calculate the fill percentages the best you can, then drive it for a couple days, and re-check the concentration.

One question, if you can only drain one gallon out, how do you expect to obtain a 50% solution in one pass? It seems like it will take more than one. Or are you trying for less than 50%?

I suppose there isn’t really a rush.

I’m assuming there some ethylene glycol leftover, and that some of the capacity is in the reservoir.

I suppose it could be low though, I guess I’ll see when I test it.

If your system is 3 gallons without including the reservoir, you might be able to use the reservoir to get the right concentration over time.
For example, if your system is 3 gallons and you can get 1.1 gallon out and your reservoir can hold 1/2 gallon (cold), you can add 1.1 gallon of concentrate into your system and 0.4 gallon concentrate and 0.1 gallon water in the emptied reservoir. Driving over time, your system will send hot coolant into the reservoir where it will mix with the higher concentration and then bring that back into your system when it cools.

That’s what I do. I always remove and clean the reservoir tank when I change coolant, anyhow. I also mark the level lines with a black permanent marker. Since it is then empty, I fill it half full of straight 100% coolant.

You need a little extra because some air bubbles will purge themselves, no matter what procedures you employ or how thoroughly you purge the system when filling it.

Disconnecting the lower hose from the radiator and connecting a garden hose to a T installed in a heater hose can enable a thorough flushing of the system. If removing the thermostat is not too difficult doing so would be preferable. When flushed and resealed, install anti-freeze equal to 1/2 the capacity of the system and top off with water and burp as instructed by the manufacturer. I would guess that my instructions would be quicker and easier than repeatedly draining and filling. Prestone offers a kit that allows pushing the water out through the radiator cap

but it never seemed as convenient as out the lower hose. There are 3 Ts included. I don’t recall ever flushing a car that required a T not included in the kit and if the T aids burping air out.

5 min at normal operating temperature.

I always remove and clean the reservoir tank when I change coolant, anyhow. I also mark the level lines with a black permanent marker. Since it is then empty, I fill it half full of straight 100% coolant.

I’ve used this method too. But one caution, only try it in the summer when freezing temperatures are impossible. It can take some time to get the coolant equally distributed throughout the system. And the 100% coolant charge in the plastic tank will freeze in very cold weather. For some reason 100% coolant has a considerably higher freezing temperature than a 50 / 50 mix.

I only do coolant changes in the summer. I should have mentioned that.
Most coolant manufacturers don’t recommend a mix stronger than 70/30 as freeze protection become counterproductive.

Besides always heeding what you’re talking about (the mixing and the freeze temperature), I’ve shared some other safety information about coolant (anti-freeze), either straight or diluted, on this forum over the years. It becomes dangerous to handle in very cold weather.

I store all my coolant in my unheated garage. Once, I was topping off coolant in one my cars at an outside temperature between zero degrees F and 32 degrees F and dribbled some on my bare fingers. The flesh turned white, immediately, instant frostbite! Liquid cools much faster than air. People can survive a long time in 60 degree air, but not so much submerged in 60 degree water.

Also, where salt is applied to snow and ice to melt it, people with pets don’t often realize when walking animals through that slush and liquid salt-water at temperatures below freezing, that pets can freeze their paws, painfully.