Just Chill: Making Sense of the Coolant Conundrum

Have you walked into an auto parts store recently to buy antifreeze? The number of options and the color-coding on the containers is enough to make your head spin. When did everything get so complicated? Here’s what’s going on.

Antifreeze—more properly called engine coolant—serves two main purposes. First, it uses ethylene glycol to extend the operating temperature of water by both lowering the freezing point and raising the boiling point. Second, because ethylene glycol oxidizes and produces acids that corrode metal, coolant contains additives that prevent acid formation and inhibit corrosion. Back when nearly all engines had cast-iron blocks and copper radiators, silicate and phosphate were the go-to corrosion-inhibiting additives. That’s what was in the ubiquitous Prestone and Zerex green coolants many of us grew up with. The antifreeze capabilities of the glycol don’t really break down, but the corrosion inhibitors do, resulting in the coolant turning a murky, rusty brown. It’s for this reason that manufacturers recommended changing the coolant every three years or 36,000 miles.

As engine designs advanced from iron blocks and heads to aluminum, and as cooling systems began using radiators with aluminum cores, and water pumps with rubber seals, it was found that the silicates in coolant, particularly when mixed with mineral-heavy tap water instead of distilled water, reacted with aluminum to form scale that could clog the coolant passages, and that phosphates could reduce the life of rubber seals. In the mid-1990s, the industry moved toward long-life coolant that used a different anti- corrosion formulation where phosphate/silicate was replaced with organic acids. Although there were publicized issues with the first of these (GM’s Dex-Cool), the industry adopted these so-called Organic Acid Technology (OAT) coolants due to the lengthened service lifetimes.

This brings us to the abbreviations like OAT. They’re confusing because the nomenclature is built around labeling the corrosion-inhibiting chemicals as inorganic or organic, but this isn’t organic in the earthy-crunchy Whole Foods way; the coolant is in no way environmentally friendly. Further, most coolant is still made with ethylene glycol and thus is still highly toxic to animals; it’ll kill them if they lap it up. Propylene glycol–based coolant is available and is much less toxic, but it supposedly doesn’t cool quite as well as the much more prevalent ethylene glycol products. Note also that coolant colors listed below are dyes, not actual chemical colors, and thus do not represent a true industry standard. But here’s a summary:

Inorganic Acid Technology (IAT) refers to old-school green silicate/phosphate coolant. This is still the advised coolant if your car has a brass or copper radiator.

Organic Acid Technology (OAT) coolants replace the silicate/phosphate corrosion inhibitors with carboxylic acids like sebacate and/or 2-EHA (carboxylic acid contains carbon and is therefore organic). OATs are typically orange in color.

Hybrid Organic Acid Technology (HOAT, yellow) coolants typically improve corrosion resistance by adding some amount of silicate (Si-HOAT—blue, red, or pink) or phosphate (P-HOAT—green or purple) to the OAT mix. The European coolants seem to be phosphate-free but with varying levels of silicate, whereas the Asian coolants tend to be the opposite—silicate-free but with a phosphate additive.

“All Makes All Models” stuff that claims to be the O negative (universal blood donor) of coolants.

So, with all that, how do you determine which coolant your car needs? Start with your owner’s manual. For example, the one for my 2008 Nissan Armada says, “Use only genuine Nissan Long Life coolant (green) or equivalent.” The clue in that is “long life.” It means “not old-school green IAT.” The manual doesn’t say which kind of OATs it eats, but if you look up green coolant on Nissan’s website, it says “contains no amines or silicates which may harm water pump seals.” If you want to find equivalent aftermarket coolant, you can use the year, make, and model coolant finders that Zerex, Prestone, and Peak all have on their websites. In my case, using Peak as an example, it pointed me to Asian green, “an ethylene glycol–based antifreeze/coolant specifically developed for use in any Asian vehicle requiring a green phosphate-enhanced organic acid technology (POAT) formulation. It contains high-quality organic acid corrosion inhibitors and is free of borate, nitrite, silicate, and amines.” Bingo.

What if you’re in a bind, need coolant, and can’t find an exact match? Treat it like you would if you were very low on oil. That is, the consequences of engine failure from lack of oil or coolant are far greater than the possibility of wear from a mismatch. It’s still coolant. It’s not like you’re pouring washer fluid into the oil reservoir. Go with “all models” coolant if necessary. Avoid putting IAT in a newer car, but if it or something else is all that’s available, use it and change the coolant when you get where you need to go.

It’s kind of maddening, though. I recently purchased three yellow-colored jugs of Prestone for the vintage cars, thinking it was what I’d been buying for decades, only to learn that it’s “all vehicles” OAT coolant. The original silicate-and-phosphate stuff is now labeled “Prestone Prime Conventional Green,” whose packaging looks like the awning of a quickie mart that sells butts, beer, and lottery tickets.

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Rob’s latest book, The Best Of The Hack Mechanic™: 35 years of hacks, kluges, and assorted automotive mayhem, is available on Amazon here. His other seven books are available here on Amazon, or you can order personally inscribed copies from Rob’s website, www.robsiegel.com.