What is the first thought that goes through your mind when you open a coolant expansion tank and instead of a clear liquid, you see a brown, foamy substance like that shown in the above image? Do you immediately suspect a blown cylinder head gasket, or are you more likely to suspect some other cause, such as for instance, contaminated coolant? Both theories have some merit but if you are new to modern automotive mechanics, you may be surprised to learn that, more often than not, many engine-cooling issues are caused by an unsuitable or inappropriate coolant mixture, as opposed to a coolant mixture that has reached the end of its useful life. Thus, in this article, we will take a closer look at engine coolants in terms of what they are, what they do, and why they should not be mixed, starting with this question-
At the risk of stating the obvious, engine coolants are designed to protect engines against corrosion while absorbing heat to shed it to the atmosphere via the radiator, and in cold climates, to prevent the coolant from freezing. However, what is not so obvious are the facts that a), modern engine coolants are the result of complex chemical engineering processes, and b), that the chemical composition of engine coolants is determined both by engine design and by environmental factors, such as the hugely different pH balances and mineral content of water in different parts of the world.
Since the interplay between these factors is a hugely complicated subject, this aspect of engine coolants falls outside the scope of this article. Nonetheless, suffice to say that for our purposes, it is necessary to understand that since modern engines contain many disparate materials, modern engine coolant formulations have to be compatible with all the metals and sealing compounds in the engine a given coolant formulation is recommended for.
In practice though, car manufacturers use about 20 different aluminium alloys, and dozens of different rubber and/or silicone-based compounds and/or polymers as sealing materials, and each combination of materials reacts differently to the water that is used to dilute the base coolant formulation to the generally accepted 50/50 concentration. We will discuss the specifics and importance of water quality later on but before we get to that, we need to understand the basic differences between the three main classes or categories of automotive engine coolants, each of which has a particular application. Below are some details-
Inorganic Acid Technology (IAT)
This is the green coolant we are all familiar with. This formulation typically contains various silicates and phosphate-based corrosion inhibitors, and it was the most commonly used engine coolant from the early 1920s to the mid-1990s when specialised aluminium alloys were first used in the manufacture of engines, radiators, and other cooling system components. Under optimal conditions, the typical lifespan of this type of coolant was about 24 months.
Organic Acid Technology (OAT)
Introduced in the mid-1990s, OAT coolants contain neither phosphates nor silicates. Instead, this type of coolant contains a variety of specialised additives and corrosion inhibitors that are chemically engineered to provide superior and consistent protection against corrosion for about 220 000 km. Note that OAT coolant is the recommended coolant for applications that include GM, VW, Honda, Mitsubishi, Nissan, Toyota, and others, and that OAT coolants come in a variety of colours*, including blue, dark green, red, and pink.
* Note that since the global aftermarket is flooded with counterfeit fluids that include engine coolants, there are no guarantees that all (or any) blue, dark green, red, and pink aftermarket coolants will conform to the same high quality standards as the blue, dark green, red, and pink coolants that are supplied by dealerships or a reputable member of the Australian Automotive Aftermarket Association.
Note that while OAT coolant formulations can be used in some vehicles dating from the mid-1990s, this is not recommended unless the vehicle manufacturer specifically approves the use of OAT coolant in the vehicle. Typical issues in non-approved applications include water pump seal failure, gasket (including cylinder head gaskets) failure, reduced boiling point temperatures, as well as poor and/or reduced corrosion protection as the result of the rapid acidification of the coolant.
Hybrid Organic Acid Technology (HOAT)
Hybrid coolant technology is relatively new and is designed primarily for use in new (late model) vehicles. Essentially, hybrid coolants are a marriage between IAT and OAT coolant technologies in the sense that these formulations contain elevated levels of silicates to protect advanced aluminium alloys, and specialised additive packages to improve corrosion protection. Note though that HOAT coolants are typically coloured orange or yellow to differentiate them from OAT coolants.
Under optimal conditions in applications that include most major European and Asian car manufacturers, HOAT coolants typically have life spans of about 240 000 km.
The above are necessarily brief descriptions of the principal differences between modern coolants, and while the increasing use of disparate materials in modern engines is one of the drivers behind the development of different coolants, it is not the main driver. The main driver is something more mundane, and it involves-
We need not delve into the complex differences between the chemical compositions of the three types of coolant here, except to say that the differences are what makes one type of coolant work in one engine, and not in another. However, despite these differences all types of coolant have one thing in common, which is the fact that all coolant formulations work best when they are mixed with distilled, but preferably with deionised water, and here is why-
For the most part, water in Europe contains high concentrations of calcium and magnesium, which creates what is known as “hard” water. In practice, these minerals combine with phosphates in engine coolants to form scale on hot metal surfaces, which inhibits, if it does not effectively prevent the transfer of heat from the engine to the coolant. Equally serious is the fact that once a layer of scale has formed, the scale prevents other corrosion inhibitors from working, which means that corrosion can progress uninhibited under the layer of scale. Therefore, manufacturers of engine coolants in Europe are legally obligated not to include any form of phosphate in their coolant formulations.
Similarly, in much of Asia, some water-borne minerals combine with silicate-based corrosion inhibitors to form a gritty substance that damages water pump seals. To prevent this from happening, Asian manufactures of engine coolants have replaced silicate-based corrosion inhibitors with phosphates and carboxylates in various relative concentrations to suit specific applications.
While there are many other examples of why it is necessary to create coolant formulations for use in specific geographical regions, limited space precludes a comprehensive discussion of this topic. Nonetheless, since it is reasonable to assume that car manufacturer will never develop metal alloys or engine coolants to suit the specific chemical composition of water in Australia, the best thing to do is to use only distilled* or deionised water when you are preparing modern engine coolant mixtures.
* Note that distillation processes do not necessarily remove all dissolved salts (magnesium etc.) or metals from water. Thus, you may use the recommended coolant for a specific application, and you may even mix the coolant in the correct proportions, but if your “distilled” water still contains dissolved minerals and/or metals, the overall efficacy of the coolant mixture will be reduced, and in some cases, it could be reduced in the worst possible way.
Therefore, the only way to ensure that the water you use is pure is to use deionised or demineralised water obtained from a reputable supplier, such as for instance, a pharmacy. Since pharmacies use this water in the preparation of some medications, it generally works well in engine coolant mixtures also, which begs this question-
We all know that engine coolants absorb heat and shed it via the radiator, but given the high temperatures modern engines generate, this is hardly a satisfactory answer. In practice, the actual processes of heat transfer, lubrication, and corrosion protection depend on both complex chemical actions/reactions and several equally complex laws of physics, but these are too complex to cover adequately in an article of this nature. However, we can simplify matters a bit, and discuss some of these processes in general terms, starting with-
Corrosion protection
In simple terms, corrosion protection occurs in one of two ways. The first involves a complex interaction between metal surfaces and corrosion inhibitors only in places where corrosion occurs. This typically applies to OAT coolants, in which the corrosion inhibitors are neutralised carboxylic acids, known as carboxylates.
In hybrid and conventional (IAT) coolants, corrosion protection is provided by phosphate-based additives that precipitate out of the coolant to form protective layers on metal surfaces throughout the cooling system.
As a practical matter, this functional difference between coolants is hugely important because the corrosion inhibitors in OAT coolants are not consumed at the same rate as they are in hybrid and IAT coolants. For instance, since the phosphates in IAT and hybrid coolants are continually deposited onto metal surfaces for as long as there are phosphates present in the coolant, the life span of especially IAT coolants rarely exceed about 2 years, and sometimes, considerably less than 2 years.
By way of contrast, the useful life of OAT coolants generally stretches over several years even under less than optimal conditions, because the corrosion inhibitors in these coolants are only activated in areas where corrosion is actually present. Thus, for as long as no corrosion is present in the cooling system, the corrosion inhibitors will remain available in full strength until corrosion does appear. In fact, lifetimes of more than 30 000 hours can easily be achieved with OAT coolants, provided only that the coolant is not contaminated or overly diluted during its recommended lifetime.
Lubrication
Lubrication to pump seals is usually provided by compounds that are either based on or derived from, molybdenum. Note though that the lubricants in engine coolants generally do not degrade or deplete to critically low levels for as long as the pH balance of the coolant remains at or above 7, or for as long as the coolant remains uncontaminated with particularly combustion gases from leaking cylinder head gaskets, which brings us to the topic of-
Although additives in engine coolants are depleted, consumed, or degraded at predictable rates by the normal processes that make coolants effective, contamination of coolants with combustion gases, engine oil, and/or unsuitable/impure water is the primary driver behind accelerated rates of additive depletion.
This is true for all types of coolants and while actual depletion rates differ based on the coolant type and the contaminant(s) involved, the principal mechanism(s) that drive additive depletion include one or more of the following-
You may have heard or read somewhere that modern engine coolants should not be mixed under any circumstances, and while this is true, it is true only up to a point. There is a lot of misinformation published on this point, and while this writer would not go as far as saying that mixing coolants recklessly will never have negative consequences, the fact is that it is possible to mix almost any coolant with almost any other coolant, and not destroy a customer's engine.
However, it must be stated that while mixing coolants should be avoided as far as is humanly possible, most, if not all manufacturers of high-quality engine coolants are in agreement that their products can be mixed with almost any other coolant, but only under the following conditions-
1) That a), the volume of the existing coolant brand/formulation in the engine is not diluted by more than a maximum of 10% by a different brand/formulation of coolant, and b), that the total volume of the (now mixed) coolant conforms to the volume specified by the affected vehicles' manufacturer.
2) That the existing coolant is well within its expected lifetime, that it is not contaminated in any way, and that its pH balance is within acceptable limits.
3) That the coolant mixture be flushed or purged from the cooling system and replaced with the recommended coolant for the affected application at the earliest possible opportunity.
Note however that while adding 10% of a different brand/formulation of coolant to existing coolant would generally not cause serious or major adverse issues if all of the above conditions are met, the resulting mixture will not perform as well as the original/recommended/ specified coolant would have done were it not diluted.
Note also that exceeding the maximum allowable 10% dilution rate with a different brand/formulation of coolant could have unpredictable results, with the degree of unpredictability depending on among other things, the coolants types that were mixed, the condition of the original coolant, and the percentage of dilution. In fact, there is no telling what might happen, but if certain additives are cancelled out or deactivated through an adverse reaction, you could expect serious and even fatal engine overheating, the failure of water pump seals, or even major coolant leaks within a matter of days as the result of the acidification of the mixture.
Given the above, the better option is always to err on the side of caution, and not to mix any engine coolant brands and/or formulations except in dire emergencies, and then only after you have educated your customer on the possible and often likely, consequences of mixing coolants, which leaves us with this-
While the basic operating principles of automotive cooling systems have remained largely unchanged from when they were first invented, the rapid advancement of engine technologies is placing increasingly strenuous demands on cooling systems. Therefore, to remain competitive, it is up to us to learn as much about modern coolants as we can and while this article does not pretend to be the last word spoken on coolant technologies, we nevertheless hope that it has inspired you to educate yourself and your customers on the need for proper maintenance of both cooling systems, and the liquids that make them effective.
