Understanding the Dangers Of Mixing Different Coolants

Learn the dangers of mixing different coolants and how it increases internal corrosion

The primary function of the engine’s coolant is to regulate the engine’s temperature, preventing it from overheating in hot weather and freezing in cold conditions. Coolant’s next most important job is to prevent corrosion within the engine and cooling system while also lubricating the water pump. Given its critical role, using the right type of coolant for your vehicle is essential. But what happens if you mix different engine coolants? Understanding the consequences can help you avoid potentially costly damage to your vehicle.

Carmakers know the types of metals used in their cooling system and the different types of plastics, rubber, and gasket materials. The coolant they choose must be compatible with all those materials.

However, not all coolant types are compatible with all metals, plastics, rubber, and gasket materials. In fact, if you top off your car’s coolant using the wrong coolant or a “universal” coolant, the additives in the different coolants can accelerate corrosion and damage the plastic, rubber, and gasket materials.

Mixing different coolants can cause serious cooling system damage

It’s not just the type of coolant that can cause problems

This example shows what can happen when you mix different coolant

if mixed with a different type; it’s also the anticorrosion additives used by the coolant manufacturer that can cause problems. For example, Toyota and Honda both use a HOAT-type coolant. However, if you mix the two types, it causes problems in your cooling system. Here’s why: when mixing different coolants, some anti-corrosion additives can cancel each other out, leaving you with no corrosion protection. In other cases, the additive packages can react, causing the coolants to gel and clog your radiator and heater core. That’s why you should never mix different types of  NEVER mix different types coolants.

The four coolant types and what makes them work

All four types of coolant contain approximately 90-95% of the base chemical ethylene glycol. The balance is the anti-corrosive, anti-foaming, anti-electrolysis, and lubricant additive package. Here are the four coolant types of additive packages.

Inorganic Additive Technology (IAT)

This conventional green antifreeze uses borates, nitrates, silicates, phosphates, and other inorganic salts to form a protective barrier at the metal surface. This process is called “passivating.” Silicate and phosphate coat internal parts rapidly, so your system is protected shortly after a coolant flush. There are two downsides to IAT coolant technology:

• Silicates are abrasive and can wear out the seals used in water pumps
• Silicates and phosphates have short lives, so the coolant must be changed every 2 years or 24,000 miles.
• Over time, the very borates, nitrates, silicates, and phosphates that provide corrosion protection deteriorate and impede heat transfer, the opposite of what you want in an engine coolant.

Organic Acid Technology (OAT) additive package

These coolants use organic acid to eliminate electrolysis (galvanic action). Most use phosphate and borate, which are silicate-free. Organic acid technology reacts chemically with metal surfaces to prevent electrolysis by forming an electromagnetic bond with the metal surface. This process takes much longer than IAT. If your vehicle uses IAT and you flush and fill with OAT, establishing corrosion protection takes several thousand miles. OAT coolants are called extended-life coolants (ELC) because they protect up to 100,000 miles and up to 10 years.

Nitrated Organic Acid Technology (NOAT) additive package

Many of the early OAT coolants also contained some level of Nitrite. These are referred to as NOAT coolants. Nitrite was added to the organic acid because it performs two functions: It is an excellent corrosion inhibitor and reduces wet cylinder wall cavitation. Cavitation occurs when the combustion inside the cylinders causes the cylinder walls to vibrate. That, in turn, causes tiny vapor bubbles to form and then collapse on cylinder walls, which slowly pick away at the metal. Nitrites coat the cylinder walls and reduce the damaging effect of cavitation.

Unfortunately, Nitrite in extended-life coolants damages aluminum engines and other aluminum components in automotive cooling systems, so carmakers no longer use NOAT.

Hybrid Organic Acid Technology (HOAT) additive package

HOAT coolants are broken into two categories: those with silicate and those with phosphate

Phosphate Hybrid Organic Acid Technology (PHOAT)

These coolants use a combination of phosphate & organic acid and are considered extended-life coolants. Asian carmakers prefer PHOAT

Silicate Hybrid Organic Acid Technology (SHOAT)

These coolants use a low dose of silicates to provide rapid passivation until the OAT can take effect. European carmakers prefer them.

The typical anti-corrosion additives used for different engine metals

Phosphate is for IRON protection and pH control
Borate is for IRON protection and pH control
Silicate is for Aluminum corrosion protection
Nitrate is for cast iron and steel protection
Mercaptobenzothiazole (MBT) and Tolytriazole (TT) are best for Copper and Brass Protection
Block Polymers are for Defoamant and Scale and Deposit Control
Potassium Soap of Dibasic Carboxylic Acid for Iron, Solder and Aluminum Protection
Potassium Soap of Monobasic Carboxylic Acid for Aluminum and Iron (w/sebacate) Protection
Nitrite for Cast Iron and Steel Protection
Molybdate for Iron Corrosion Protection (w/nitrite)
Tolytriazole for Copper Corrosion Protection
Modified Silicone for Defoamant

Coolant color

IAT (Inorganic Acid Technology) is typically bright green
OAT (Organic Acid Technology) can be Orange, Red, Blue, or Dark Green
HOAT (Hybrid Organic Acid Technology) is Yellow, Turquoise, Pink, Blue, or Purple
Dex-Cool (An OAT sub-type. Do NOT mix with any other type) Orange

Warnings from the experts about mixing different coolants

Gates, maker of coolant hoses, fittings, and water pumps, publishes several bulletins for technicians on the practice of mixing different types of coolants. Here’s a warning from Gates Bulletin TT002-13:

Gates warning about mixing different coolant types

“Corrosion inhibitors and additives from one coolant can negate the effects of inhibitors in another coolant. Corrosion can take over well before 5000 miles, and then the car is back in the shop with a leaking water pump and clogged radiator and heater core.

Topping off the system is a common practice at many shops. This can begin the corrosion process unless the proper coolant is used. Most shops use Universal antifreeze because it says on the container that mixing with any color coolant is OK. But take a look at the back of the bottle. They all recommend or require a drain, flush, and fill to experience the expected corrosion protection.”

Mixing different types of antifreeze can reduce their corrosion protection and lead to corrosion problems. Engine manufacturers recommend a 10% limit on mixing coolant types. If you add more than 10%, it is recommended that you flush the system and replace the antifreeze—EET Corporation

Gates Bulletin TT002-13

Then there are these manufacturer quotes from Aftermarket Business, July 2006

Excerpts from: An antifreeze antidote

Ford warning about mixing different coolant types

Ford doesn’t believe a universal formula could work properly in all vehicles by distributing a Q&A document to their dealer network and other public channels. It says, “Due to the complexity of cooling systems, no one coolant has been proven to work in all vehicles,” and “use of non-approved ‘universal’ engine coolants may lead to eventual engine damage or failure. Problems caused by the use of non-approved ‘universal’ coolants may not arise immediately but may occur over longer periods due to additive compatibility issues.” Conville says they do not have specific test results on such coolants because “there are many different coolants in the North American aftermarket and Ford does not have the resources to test them all.”

“John Conville, a coolant expert at Ford Motor Co., emphasizes that Ford does not recommend the use of any specific aftermarket coolants in their vehicles (unless approved). “…the only aftermarket coolants that Ford recommends are the appropriate Motorcraft brand engine coolants.” With the use of non-approved coolants, Ford is concerned about “corrosion of metals and incompatibility with some of the polymeric materials we use in our vehicles.”

GM warning about mixing different coolant types

Consumers need to be careful about what they buy because it’s a chemical manufacturer’s job to sell coolant, and they “enhance their ability to sell coolant by claiming that a coolant is compatible with all the carmakers. We know from the formula that some of them don’t work.”   GM spokesperson Lockwood

Chrysler warning about mixing different coolant types

Chrysler states: “Mixing of coolants other than those specified (i.e. non-HOAT) may result in engine damage that may not be covered under the new vehicle warranty, and decreased corrosion protection.

Warnings from auto magazine- Aftermarket Business

If a non-HOAT coolant is introduced to the cooling system in an emergency, it should be replaced with the specified coolant as soon as possible.” An antifreeze antidote, Aftermarket Business, July 2006

Toyota warning about mixing different coolant types

Toyota, which has a long life and super long life non-silicated products, too, does not advocate the use of non-genuine antifreeze/coolants. Toyota Motor Sales’ Bill Kwong in product communications explains that other formulas tested didn’t provide the same performance. “Evaluations of competitive formulations have been made in simulation and field tests, showing (Toyota’s) product superior in water pump seal protection, leakage and deposit formation.” It’s uncertain whether he’s referring to competitive OEM or aftermarket formulas.

Valvoline, maker of Zerex Antifreeze — on mixing coolant types

Some aftermarket chemical makers share stances similar to the OEMs. The Valvoline Co., a division of Ashland Inc. and the maker of Zerex® antifreeze products, recommends following the OEM’s formulation suggestions. Valvoline’s David Turcotte, technical director of the company’s Technology and Product Development division, believes that in some instances of damage, “cause and effect isn’t always obvious.” Other than the apparent compromises in performance, much of the physical damage caused by improper coolant use is not noticed until the cooling system is dismantled, said Turcotte at a recent product presentation.

What antifreeze should you use?

Aftermarket “Universal” coolants sell for about $10/gallon. Genuine OEM coolant sells for around $23/gallon. Most cooling systems require no more than two gallons. So using the OEM antifreeze means a trip to the dealer and an extra cost of around $20. Replacing a heater core due to premature corrosion can cost upwards of $1,500 and an average radiator replacement runs about $450. Water pump replacements can easily cost $400.

To me, it’s a no-brainer. But if you still have doubts, do the math.

© 2015 Rick Muscoplat