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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
That was nice, but the base is not the reason you can't mix different types of antifreeze.
- 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
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.