Bogus or not? Coolant won't mix in engine

I’ve heard that when you change out coolant, you should mix it 50/50 first. I mean instead of just pouring in 100% coolant, and adding the same amount of water. I realize there’s a practical problem if you try it the second way, as you don’t always know how much you need to add. So from that perspective the practical sol’n is to mix first, then add. But let’s assume you know exactly how much you need to add to fill the system for purpose of discussion.

So what I’ve heard is that if you add 100% coolant, then afterward the same amount of pure water, that’s a big problem, b/c the pure coolant will never mix with the water in the engine. The coolant is heavier, so – if you believe this idea I’ve heard – the pure coolant will go to the bottom of the engine and never leave. It will never mix w/the pure water. The pure water will stay on top, and will corrode the top portion of the engine.

I say this is doubtful, even bogus. It may not mix well for some time, like right after you do it and the engine is cool. The pure coolant will be at the bottom of the radiator. But it seems to me as soon as the thermostat opens, the coolant at the bottom of the radiator will begin to circulate everywhere in the engine, and I can’t believe it would take more than a few hours of driving at full operating temp before the coolant would be fully mixed with the water, just as good as if you had premixed it before adding. It’s a closed system. Coolant is circulating everywhere There’s nowhere for the pure coolant to hide.

What do you think? Will the 100% pure coolant remain at the bottom of the engine forever, and cause the top portion of the engine to rust inside out? If it eventually does mix, how long does it take? A month? A week? An hour? How long will it take for the pure coolant to mix evenly with the rest of what is in there?

100% BS. Guaranteed. Absolutely. An engine is a GREAT way of mixing coolant. I always add the antifreeze first, to make sure the correct amount is added.

What nonsense…

Agree, its not like oil and water. If you want to test it, take a little antifreeze in a glass jar and add some water. It’ll mix instantly just like Kool Aide.

Welllll, not just like Kool Aide. Don’t drink it,

As soon as you add water to pure antifreeze or vise-versa, it mixes instantly.


“the pure coolant will go to the bottom of the engine and never leave”

It’s a forced water cooling jacket. There is no area where coolant can sit. How do you think it could cool the engine if it did?

Me thinks maybe someone was having fun with him like sort out the left hand from the right hand nails and he just fell for it and never figured it out. Someone is probably still laughing about it.

Coolant manufacturers know what they are doing. “Requiring” premixing means the coolant is more prone to settling out causing problems later. Think of it. There are many fluids that have the enhanced ability to stay mixed. Coolant is one, two stroke motor oil is another. But, having said this. It does illustrate that you should not mix coolants in general as this capability could be compromised. IMHO, that is much more important then worrying about pre mixing.

George, I hope you can remember who originally gave you that advice. Now you know how to handle further advice (of any kind) from the same party.

+1 with all the other comments. An engine is like a huge blender.

I wonder how it works with MaiTais…

"So what I’ve heard is that if you add 100% coolant, then afterward the same amount of pure water, that’s a big problem, b/c the pure coolant will never mix with the water in the engine. "

Where did you “hear” this BS?

Bogus or not? Coolant won't mix in engine.

What discount autoparts store clerk told you that?? It’s totally bogus.

First off…20 years ago you couldn’t buy 50/50 mixture (at least not where I lived). You had to dilute it yourself.

Second - if the stuff wouldn’t mix…then the fluid manufacturers have a lot of explaining to do by selling the non-mixed stuff.

After reading this, I tried an experiment. I usually mix myself a bourbon and club soda before supper. However, I decided to drink the bourbon straight and then drink the club soda and let it mix in my stomach. I think it mixed up just fine, although I find that I just sipping the bourbon and forgetting the club soda.
Of course, the coolant and the water mix right away.

@MikeInNH made a good point. Was the clerk at the parts store, who only knows parts by what his computer says, trying to sell you on 50-50 pre-mixed antifreeze? The mark-up on the water content is more than Evian. Pre-mixed antifreeze should be outlawed as stupid and way too expensive.

Why I never buy the 50-50 mix to change coolant.- It is very difficult to get all the coolant out of a modern car. They don/t have petcocks or even drain bolts on blocks anymore so you can’t get everything out, however you can get 50 % out without removing the thermostat. I usually pull the bottom rad hose and one heater hose then drain all I can. Next, flush with cold water until it runs clear. Let drain and close up. Refill with pure coolant to 1/2 capacity then fill the rest with water. keep a jug of water in the car fr a few days to top off with.

hmmm … where did I hear this? I Ok, I had to put my thinking cap on. I’m not sure if this is where, but how about this? I’ve found the other info on this site very good. I’m inclined to believe what they say, but like I say was doubtful about this one. From what I heard from the group here on CarTalk, you folks seem doubtful too. So I’m unsure.

Seems easy to decide, one way or the other. It’s science after all. Anybody actually done the experiment? Drain everything out, then put in pure coolant and pure water into a cold engine, and every 30 minutes of drives, take a sample, measure the specific gravity?

"Coolant is much heavier than water and will NOT mix in the engine block.

Coolant flow in an engine is through the cylinder heads. The lower block fills with liquid but does not flow. Concentrated coolant will enter this area and remain there, because of its weight. Water with a small amount of coolant will circulate through the upper part of such a system. This promotes corrosion as the mixture is inadequate to protect the metal in the system."

That part about the coolant flow only being through the cylinder heads and not through the engine block is totally bogus. Who came up with that swill?

It might be out of context. You can look at the entire thing by clicking the link. I expect what is meant is that during warm up, before the thermostat opens, the coolant flows in the upper part of the engine more than in the lower part. Actually, I have no idea how much coolant flows in the upper part compared to the lower part, irrespective of whether the thermostat is open or not. That’s a mystery to me. But the water pump is usually near the top of the engine, and heat rises, and oil is a pretty good coolant for the lower part of the engine, so one could imagine there may be more water flow up than down.

Guy’s absolutely wrong, for all the reasons noted above. It is SLIGHTLY denser than water, 1.11 g/cm3 vs. 1.0 g/cm3 for water, but is it MISCBLE with water, meaning that it mixes at all concentrations completely with water, just like alcohol. And the statement that ‘the lower block fills with liquid but does not flow’ is PATENT NONSENSE. The water pump is pushing coolant throughout all passages in the engine, and their ‘tortuosity’ (how the bend, split, and recombine) is PERFECT for the quick mixing of water and antifreeze, once the thermostat opens.

Makes me wonder what other nonsense is on his site.

edit - I just did a quick check, other questions are answered reasonably, so now I wonder how he got this one thing so wrong…

I suppose adding them separately works well if you know exactly how much the cooling system will hold.