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Though many don't realize it, choosing the proper engine coolant for your car can be a mission fraught with danger -- picking the wrong coolant can shorten the life of your engine in many ways. So read up, and be prepared.

First, the short explanation: if you haven't had any coolant-related cooling problems, and the coolant in the engine is not foamy or brown, and you know what kind of coolant is currently in the engine, use the same coolant. Mix it 50/50 with de-ionized water. If you are having a coolant emergency, mixing different types of antifreeze is usually better than just putting water in the system, as long as you thoroughly flush the system as soon as possible.

First, a bit of technical background. Mixing antifreeze with the water in your cooling system serves two purposes. The main purpose, as the name suggests, is to ensure that the water doesn't freeze -- the antifreeze raises the boiling point and lowers the freezing point of the water in the cooling system. It also provides lubrication and corrosion protection for the different components of the cooling system (engine block, water pump, radiator, etc). It used to be that there was one kind of antifreeze, the green (or greenish gold) stuff, and you dumped in some of that with some water and everything was fine. Now, it's not so simple, because there are several different types of antifreeze out there which (as a general rule) should not be mixed. To confuse matters further, the different types of antifreeze are different colors, but the colors are added by the manufacturers and are not a reliable indication of what type of antifreeze you are actually getting. Some manufacturers, such as Chrysler, use different coolant formulations in different models but dye them all the same color -- and using the wrong coolant will void your warranty. So forget the color. Instead figure out what is actually in the coolant.

Virtually all antifreeze products are either ethylene or propylene glycol based. Ethylene glycol or propylene glycol give antifreeze its high boiling point and low freezing point. They do not provide lubrication or corrosion protection.
  • Ethylene glycol has been the active ingredient in most antifreezes for years, and will probably be out there for many more to come. Ethylene glycol antifreeze straight from the bottle will freeze at about 8oF (-13oC) and boil at about 330oF (166oC), but it carries heat about 15% less efficiently than water. When mixed 50/50 with water, the compromise is good enough for most cars and driving conditions. It can be mixed up to 70% coolant for better boiling/freezing performance but the heat transfer ability will be reduced slightly. Mixtures of less than 50% coolant or more than 70% coolant are not recommended.
  • Propylene Glycol is commonly used as a replacement for ethylene glycol that is less dangerous to animals (it is used in Prestone LowTox and Safe Brands' Sierra brand antifreezes, among others). While its main marketing advantage is that it is considered nontoxic to animals, it also provides good cooling performance. Straight from the bottle, a propylene glycol antifreeze will boil at about 370oF (188oC) and freeze at around -70oF (-57oC). When mixed 50/50 with water, it will boil at about 256oF (124oC) and freeze around -26oF (-32oC). Propylene glycol also conducts heat better than ethylene glycol and is sometimes used by itself (with no water) in racing applications. Propylene glycol can be mixed with ethylene glycol with no damaging side effects. However, a mixture of the two will not be nontoxic to animals, and since propylene glycol coolants are not meant for extended service intervals, mixing it with a long-life ethylene glycol coolant will reduce the service life of the coolant mixture.
That was nice, but the base is not the reason you can't mix different types of antifreeze.

The next step is corrosion protection. This is where antifreeze starts getting weird and incompatible. The engine coolant is in constant contact with metal parts of the engine and radiator, so something must be done to prevent it from corroding the metal surfaces, which may be iron, brass, steel, aluminum, or copper.
  • Conventional American coolants that are green or green/gold will contain amines and some inorganic salts of borate, phosphate, or silicate, as well as nitrites, nitrates, and amine additives to prevent rust and corrosion. These salts make the coolant somewhat alkaline, with a pH around 10 or 11. Silicates form a coating on metal surfaces that prevents rust; they are especially effective on aluminum. Silicates also protect against cavitation erosion, which occurs when the coolant starts to boil under high pressure. As the coolant enters the water pump, it starts to boil and create bubbles -- at the outlet of the pump, the bubbles collapse with explosive force and the mini-explosions can pockmark the pump chamber, knocking off the protective layer of corrosion inhibitor. Silicates act quickly to recoat the exposed surfaces. However, some of the additives are abrasive, and others, such as the silicates, can become unstable and drop out of solution to form a gel. Also, as glycol degrades over time, it forms acids which can cause corrosion. Because of this, there must be enough of the alkaline corrosion inhibitors to neutralize the acid; this is called 'reserve alkalinity' and varies depending on the type and quantity of additives. However over time, heat, dissolved oxygen, minerals in the water, and reactions with the metal surfaces of the engine deplete the additives -- you must change the coolant before this happens. It is especially important when there are different types of metals present, such as aluminum and iron. The aluminum and iron act as like the terminals of a battery; as the coolant degrades and becomes acidic, it conducts electricity between aluminum parts such as the heater core or radiator (they would become the anode) and the iron engine block (the cathode), which accelerates corrosion, which creates more acid, etc. Don't let this happen. An exhaust leak into the coolant, such as from a faulty head gasket, will also very quickly cause the coolant mixture to become acidic.
    Water that contains high amounts of calcium and magnesium ('hard water') can react with phosphates and form sediment and scale. This is why many European vehicle manufacturers use antifreeze that contains no phosphates (hard water is more common in European countries). Phosphate free antifreeze may be blue, yellow, or pink. Do not use softened water in your cooling system! Water softeners replace calcium with sodium, and sodium is corrosive to all metals in the engine.
  • Organic Acid Technology (OAT) is one of the latest types of corrosion protection used in coolant and is somewhat popular among car manufacturers for use as stock coolant in new engines. Antifreezes containing OAT use organic acid salts to prevent metal corrosion. Pure OAT coolants contain no silicates or phosphates. Azoles such as tolytriazole are often added to help prevent corrosion of copper components. Generally there will be a pair of synergistic acids in the coolant that combine to form carboxylates. These deplete very slowly, thus eliminating the need for traditional additives and frequency fluid changes. Coolants that use OAT also tend to be less alkaline than the traditional coolants, and often have a pH of around 8-8.5. OAT based coolants tend to have an orange, pink, or red dye to distinguish them from other types of coolants. OAT based coolant have a longer service life than glycol based coolants, and are marketed as having a service life of 5 years/150,000 miles. OAT based antifreeze should not be mixed with the more traditional antifreeze, or the corrosion protection and service life will be reduced.
  • Conventional Japanese coolants, which are generally green or red, contain no silicates but do have a lot of phosphates and other inorganic salts, and may also contain some organic acids.
  • Conventional European coolants, often blue or yellow, contain some silicates and sometimes inorganic salts, but no phosphates. Some also contain an organic acid.
  • Hybrid European coolants, sometimes blue or green, are similar to the older conventional European coolants but have more organic acids. Silicates primarily protect aluminum, and organic acids protect other metals and provide longer-lasting protection for the aluminum.
  • Hybrid American coolants, which may be green or orange, contain a fair amount of silicates, and a blend of organic acids.

So enough boring technical stuff; what's the answer? What to use in your car?
Again, the first advice is to try to use whatever was in there before; use a 50/50 mix of coolant and de-ionized water unless you spend lots of time in extreme temperatures (in which case you should use up to 70% coolant and 30% water). If you don't know or can't find what was put in there before, or if you have some concerns about your cooling system, or if you just want to change, here are some guidelines:
  • If your car had a hybrid or conventional European type antifreeze, a conventional American silicated antifreeze is fine.
  • If your car had a pure OAT antifreeze in it before, replace it with a pure OAT antifreeze such as Dex-Cool.
  • Your aluminum radiator will last longest with an OAT coolant. Though OAT coolants contain good protection for copper and brass, they do not protect the lead solder used to put them together.
  • If you don't know what was in the car, or want to change it, buy a bottle of radiator flushing fluid and follow the directions to make sure that all of the old fluid is out of the system before introducing the new fluid. Monitor the new fluid closely over the following week, and if it starts to look brown or foamy, flush the system again and refill with the desired coolant.
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