Engines are complex machines with rapidly moving metal parts and small clearances. And as such, they generate a lot of heat. You already know that coolant is an integral part of keeping your engine from exploding, and you probably also know that you have multiple coolant options that aren’t interchangeable. But how do you know which is OK to use? Color is not a reliable indicator in choosing between OAT, IAT and HOAT, so you better choose wisely … and what is HOAT coolant anyway? Let’s dig in.
1. The Story on HOAT
Inorganic Acid Technology (IAT) Coolant was pretty much the original type of coolant and is still used in older vehicles. It lasts around 30,000 miles before it turns too acidic and needs to be changed. Organic Acid Technology (OAT) Coolant uses additives to increase its lifespan significantly — up to around 150,000 miles. Hybrid Organic Acid Technology (HOAT) Coolant is a hybrid of the two.
2. How HOAT Came to Life
You want your coolant to do a few things. Most importantly, it cools the engine, but remember it’s also passing through metal passageways and subject to vastly different operating temperatures, so it needs to deal with all of that. The original IAT is strongly anti-corrosive, but it needs to be changed regularly. OAT lasts much longer, but it doesn’t defend against corrosion as well. Ironically, simply mixing the two can have damaging corrosive effects, but the hybrid-engineered HOAT uses a chemical composition that keeps rust and buildup at bay, is effective at very low and high temperatures alike, and lasts five years and beyond.
3. Careful, HOAT Doesn’t Play Nice
Nope! On the surface, it seems like a hybrid between IAT and OAT would be compatible with either, but that’s not the case. They may share similar names, but the chemical recipe is different enough that manufacturers do not recommend mixing them. Doing so potentially limits your coolant’s effectiveness in doing its job and can greatly reduce the lifespan. And as mentioned earlier, it might actually gunk up the system. This is true even if you’ve drained the coolant from an engine — it’s not so easy as to just add a different type because traces of the old type remain and cling to passageways.
4. When to Use HOAT
As with most other questions about your car, check your owner’s manual. It should specify the coolant type. You may think the benefits of IAT, OAT or HOAT are what your vehicle needs, but it’s not a call you can easily make without cleaning out the entire cooling system (more than just a flush). So it’s really best to just stick with what was designed to support your engine and you will be fine.
Always wait for an engine to cool down before opening a radiator or coolant reservoir cap, as the system is under pressure and the coolant is scalding hot. And remember, contrary to popular belief, color should not be the determining factor for adding or mixing coolant because it is not standardized or indicative of chemical makeup. Always check with the manufacturer to make sure you’re adding what your engine needs.
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Photos courtesy of Blair Lampe.
Blair Lampe is a New York-based professional mechanic, blogger, theater technician, and speechwriter. In her downtime she enjoys backpacking wherever her boots will carry her, rock climbing, experimental theatre, a crisp rosé , and showering love on her 2001 Sierra truck.