Coolant Mix and Overheating

The problem:

Sometimes a water-cooled engine such as the Rotax 582 seems to run hotter than it should, for no apparent reason. An identical engine installed the same way in another plane may be fine, but one runs hot, and the other doesn't. This problem can cause no small amount of hand-wringing and head scratching, but as mysterious as it may seem, it may be quite simple to fix.

Now, more precisely, the situation we're talking about is this: the coolant temperatures may run up to the 180 - 190 F range as opposed to the more reasonable 165 - 175 F. Our radiator may be no smaller than someone else's that works just fine, and the thermostat is not stuck or obstructed. It just runs hot.

The problem might be that the coolant mix is too rich. That is, too much "antifreeze" and not enough water. And to no one's surprise, what you do to fix it is take out some of the antifreeze and replace it with water. Reducing the mix ratio can easily lower the temperature by 15 F.

Why a rich mix gives less cooling:

First of all, in this article, we refer to the stuff you buy simply as “antifreeze.” It comes under many different trade names, and for automotive applications, is usually ethylene glycol. We'll just call it “antifreeze.”

Now if you've ever poured the stuff into a radiator, you no doubt observed that it is much “syrup-i-er” than water. That is, it has a much higher viscosity. This means that it will not flow as fast through the system as water, and the cooling effect will be less.

Further, the specific heat of antifreeze is less than that of water. This means that for a given temperature change, the antifreeze will carry less heat each time it circulates from engine to radiator and back.

So, if the mix is too rich, the cooling efficiency will be reduced because (1) the flow rate is less, and (2) less heat is transferred per cycle of the coolant.

The manufacturer's recommendation:

The standard recommendation is to use a 50/50 mix of antifreeze and water. This represents a compromise between cooling efficiency and the ability to prevent the mix from freezing during cold weather. After all, the initial purpose of antifreeze is to prevent freezing.

But a 50/50 mix does not give the best cooling. For improved cooling in hot weather, we should use less antifreeze and more water, perhaps going to a 25/75 or a 20/80 mix ratio.

Everyone knows antifreeze prevents corrosion, and corrosion is bad. True enough. But, even a 10/90 ratio of antifreeze to water will serve the puproses of inhibiting corrosion, at least according to one source of information.

More is better!

Not in this case. When you're adding the coolant to an engine, especially to an aircraft engine, you want to do it as good as it can be done. Cut no corners! Don't use just a little if a lot will do. After all, the stuff coming out of the jug looks high-tech and scientific, and it's expensive, so it must be good. Therefore, use a lot!

Yes, the stuff is good, high-tech, and all that, but it's physical properties are not as good as plain, pure water when viewed from the point of cooling. Therefore, you must resist the temptation to put in more just because more seems like it should be better.


Temperature drops ranging from 10 - 15 F are typically obtained by decreasing the mix from 50/50 down to 25/75 or perhaps 20/80. It is not possible to give precise numbers here because of uncertainty in knowing exactly what the initial mix was, and so forth.

In one instance, the original mix in a newly-purchased plane appeared to be pure antifreeze. When the mix was changed to 25/75, the temperatures dropped by 20 F! Here's the email that reported the results:

Hi Doc,

Well it worked perfectly and I got a 20 degree reduction in water temp by decreasing the mix to 25/75. By the looks of the stuff that came out, I really think that it was full strength antifreeze.

Unfortunately it will not be running any time soon. I had an engine failure due to a poor gas tank vent design and didn't see a two strand barbwire fence on the landing. I caught the left landing gear on a post and ripped it off. Needless to say, the rest is self evident. But then that is another story.

Thanks Again,

Adjusting the mix, starting from scratch:

Determining the amounts of antifreeze and water is not particularly difficult if you're filling a completely empty cooling system. The following amounts are calculated for a system whose total volume is 8 units, whether pints, liters, quarts, or whatever. (The total volume of a Rotax 582 system is about 8 pints in typical installations.)

    Mix Ratio         Antifreeze         Water

        50/50                    4                      4
        40/60                 3.2                   4.8
        33/66                 2.6                   5.4
        25/75                 2.0                   6.0
        20/80                 1.6                   6.4
        10/90                 0.8                   7.2

If your system capacity is more than 8 units, say 10 for example, just multiply each number in the table above by 10/8 = 1.25.

If your system capacity is less than 8 units, say 6 for example, just multiply each number in the table above by 6/8 = 0.75.

Reducing the mix, starting with a 50/50 mix ratio:

If the system is already filled with coolant, what you must do is drain part of the mix and replace it with pure water. Based on a system whose total volume is 8 units, here are the amounts to drain in order to lower the mix ratio from 50/50:

    New Mix Ratio         Drain and Replace

            40/60                           1.6
            33/66                           2.75
            25/75                           4.0
            20/80                           4.8
            10/90                           6.4

If your system capacity is more or less than 8 units, you can adjust these numbers the same way as described above.

Reducing the Mix, general case:

This involves draining a fraction of the coolant and replacing it with pure water as before. The amount to be drained out can be calculated if we know the mix ratio of the coolant presently in the system and the total system volume. The calculation is illustrated in the following.

First, express the mix ratio as a percentage, like this:

    Mix Ratio         Percentage

        50/50                   50
        40/60                   40
        25/75                   25       .... and so forth.

We have two different mix ratios to deal with. One is the mix ratio of the coolant in the system now. We'll call this the OLD mix ratio.

The other is the mix ratio we're trying to get. We'll call it NEW.

The formula for finding how much to drain and replace with water is

    Drain = TotVol x ( 1 -- NEW / OLD )

where Drain is the amount to be drained out, and TotVol is the total volume of the cooling system.


Suppose we have a system whose total volume is 10 pints and the mix ratio of the coolant in it now is 60/40.

That is, it has more antifreeze in it than water. We want to reduce the mix to 20/80. How much of the coolant should we drain and replace with water?

    Mix ratios, in percent:         OLD = 60         NEW = 20

    TotVol = 10 pints

    Drain = TotVol x ( 1 -- NEW / OLD )

      = ( 10 pints ) x ( 1 -- 20 / 60 )

      = ( 10 pints ) x ( 1 -- 0.333 ) = ( 10 pints ) x 0.667

      = 6.67 pints

Therefore, if we drain 6.67 pints of the original coolant and replace it with pure water, the new mix ratio will be 20/80. And that's it.

An obvious final comment:

If you live in an area where the temperatures fall below freezing, you will need to increase the mix back to a higher value before winter sets in. Which is to say, you will need the antifreeze to prevent the system from freezing up. But in cooler weather, it is unlikely that overheating will be a problem. It would be a shame to forget to change the mix.

Credits: Thanks to my instructor and flying buddy, Mr. Ted Clement, for sharing this information. He's a gentleman, a scholar, and he doesn't fly all that bad either.

Author:   Doc Green