Other Articles - October/November - 2018

Cool It: What You Need to Know about Your Vehicle’s Cooling System

A recent study found that up to 40% of all engine failures were a direct result of cooling system issues. As the vehicles we service continue to evolve, the coolants and systems designed to keep our transmissions and engines operating at the correct temperature are also changing. Many of the changes have led to confusion at the repair shop level.

For decades, vehicles used ethylene glycol (green) coolant that simply circulated the coolant throughout the cooling system. The temperature of the transmission and engine were controlled by the same coolant and cooling system. Today, many applications use separate systems for the engine verses the transmission.

When it comes to coolant, the choices are numerous. Making a mistake with something as simple as your choice of coolant can lead to major cooling system issues. Adding the wrong coolant can lead to component corrosion, failure, or gelling issues. Both American and European original equipment manufacturers (OEMs) warn about using the wrong coolant as they have identified plastic cooling system component failure, gasket, and corrosion issues.


A coolant must:

  • Absorb and transfer heat
  • Raise the boiling point
  • Lower the freezing point
  • Prevent corrosion, cavitation, and galvanic reactions
  • Lubricate and protect cooling system seals and gaskets

The American Society of Testing and Measurements (ASTM) establishes standards for coolants, while Europe has its own certification process and standards. Most automotive coolants are polyol based with ethylene, glycerol or propylene added to create the antifreeze effect.

Where the coolants really start to vary and where the problems arise is in their additive packages. Manufacturers have failed to agree on which is the best technology, so each has gone its own way, leading to major confusion in our industry.

Additive packages will typically include silicates, phosphates (light duty and passenger cars), or nitrates, molybdates (heavy duty and diesel applications). Several different coolant types and additive packages are used, including:

  • HOAT (Hybrid Organic Acid Technology)
  • IAT (Inorganic Acid Technology)
  • NOAT (Nitrate Organic Acid Technology)
  • OAT (Organic Acid Technology)
  • SiOAT (Silicone Organic Acid Technology)
  • PHOAT (Phosphate Hybrid Organic Acid Technology)
  • Glysatin (G48)

It would seem logical that if there were major differences in the coolants, the manufacturers would simply add dye to change the color, making it simple for the customer to get the correct coolant. While they do dye their coolants, the color really doesn’t guarantee you’re selecting the correct coolant, as the OEM determines the color and it doesn’t necessarily denote the additive package. Many colors are used, including green, blue, red, orange, yellow, pink, and purple, to name a few.

So what do you do when you visit your local parts store and you’re faced with 10 different choices? Some would lead you to believe that you can use a coolant listed as universal in any vehicle, no matter if it matches the OEM chemical technology or not. The vehicle manufacturers would beg to differ on this point and, as with other technology, you must choose which path to follow. As you can see, it’s important to educate yourself to provide the best service for your customer.

IAT — This is the old-style, green coolant used by American and Asian applications. European applications were typically dyed blue. IAT was used by GM up to 1996, Ford up to 2002-2003, and Chrysler up to 2001. IAT uses a phosphate-silicate additive package that provides good protection for cast iron, aluminum, brass, and copper parts, but its life expectancy is lower than the extended life coolants. Typical life expectancy is in the 2-year/30,000-mile range.

OAT (Figure 1) — OAT technology doesn’t use phosphates or silicate additive packages as in IAT coolants. Corrosion inhibitors such as sebacate and 2-ethylhexanoic acid are used instead. This type of additive package lasts much longer than those used in IAT coolant.

The most common type of OAT coolant is Dex Cool, which is dyed orange. Other manufacturers, such as late-model VW/Audi (pink or red), late model Dodge/Chrysler (purple) and Honda (dark green) also use OAT technology. OAT won’t protect brass and copper components used in earlier model cooling systems. Some diesel applications require HD OAT coolant as it is nitrate free.

OAT is an extended life coolant with a life expectancy of 5 years/150,000 miles. Most universal coolants are OAT designs.

HOAT (Figure 2) — HOAT technology contains organic acids and several inorganic additives. HOAT applications typically contain silicate. This works great for aluminum engine and cooling system components. The most common types of HOAT fall into specifications G0-5, G-11 or G-12 (typically yellow, orange, purple or pink), which is recommended by many manufacturers such as Chrysler, Ford, Mercedes, BMW, Volvo and Mini Cooper applications. Generally, this type of coolant has a life expectancy of up to 10 years/180,000 miles.

PHOAT (Figure 3) — The difference between HOAT and PHOAT is the primary additive component, which uses phosphates rather than silicate. Honda, Toyota, Kia, Hyundai, Subaru, and Nissan typically recommend this type of coolant. Many of the aftermarket coolant manufacturers will specify “For Asian Vehicles” on the container and it is often red or blue in color.

SIOAT — This application basically adds silicates to the coolant additive package. It is used by VW/ Audi (G-12++, G-13) Mercedes (325.5 pink) and it is one of the newest coolant designs on the market.

Glysatin — Glysatin (G48) was developed primarily for the European markets and is recommended for several applications such as BMW, Mercedes, VW/Skoda/Seat/ Porsche, and Opel/Saab. G48 is an ethylene glycol product that uses 2-ethylhexanoic acid, sodium salt and is nitrate, amine, and phosphate free. It is typically dyed blue or green.


Similar to the light-duty market, heavy-duty and diesel engine manufacturers have developed their own additive packages. Some applications require a NOAT while others require a nitrate free OAT coolant.

Diesel engine cooling systems have different requirements when compared to gasoline applications. Some extended-life coolants are approved by the OEMs for both gas and diesel applications, but many aren’t.

In diesel applications, the combustion process causes the liners in the engine to echo similar to a bell during combustion. This results in a phenomenon known as cavitation, producing tiny vapor bubbles. If the bubbles implode, it can create enough force to erode the liner walls and other cooling system components.

To address this issue, the additive package is different from conventional antifreeze. Known as a Supplemental Coolant Additive (SCA), the “pre-charge” enables the coolant to address the unique requirements of a diesel cooling system. Most diesel-approved coolants contain the SCA pre-charge while some require you to add the pre-charge to the coolant (Figure 4).


As you know, full-strength coolant must combine with water to function properly. You many not be aware that hard water has a major negative impact on the additive package’s effectiveness. All coolant manufacturers use distilled/de-ionized water in their premixed formulations. All coolant manufacturers and OEMs recommend the use of distilled/de-ionized water rather than tap water to mix with full-strength coolants.


Like the fuel in your tank, the additive package in your coolant is consumed during use. This means you need to replace your coolant, as its effectiveness declines with use. Many fleets sample their coolant on a regular basis to reduce the chance of durability issues. On diesel applications, the SCA pre-charge is available at your local parts store as an additive.


As your coolant additive package starts to deteriorate, the coolant can become conductive. Current starts to travel though the cooling system leading to metal transfer and corrosion; this is called a galvanic reaction.

The more the coolant deteriorates, the worse the galvanic reaction and component damage will be. A galvanic reaction will cause damage to cooling system components, such as the heater core, transmission cooler, intake manifold, cylinder heads, and some radiator materials.

To check for conductivity within your cooling system, place a voltmeter lead into the coolant (coolant cold) and connect the other lead to the battery negative terminal (Figure 5). With the engine running, check the meter reading. If reading is greater than 0.1 volts (100m V) a galvanic reaction is occurring.

Most manufacturers suggest readings above 0.3 volts (300mV) indicate a significant galvanic reaction is occurring and action must be taken. Check, clean, and repair the electrical system grounds and flush or change the coolant.


Topping off the coolant can lead to a lot of confusion. As we discussed earlier, OEMs and many coolant manufacturers issue warnings regarding the use of the wrong coolant. Some coolant types can be safely intermixed while others can’t. Intermixing the wrong combinations can cause many problems, including turning the coolant to a substance that looks like goo. In addition, seal and rubber part failure can occur.

But don’t panic. If you’re adding a small amount to top off your cooling system, it probably won’t cause problems. But if you’re adding a significant amount of coolant, it may be a different story. To decrease the confusion, refer to the chart (Figure 6) or to the vehicle’s service information.


Some hybrid applications control the temperature of the hybrid components by using the vehicle’s A/C system while others use a cooling system similar to those used for a conventional application. Still others use a combination of the A/C system and a standard cooling system.

On those applications that use a cooling system, many will have a separate reservoir for the hybrid electronics and transmission system. Some hybrid applications require the use of OE coolants.


As we are all aware, government fuel economy standards are a major issue that all major manufacturers face. One of the simplest ways to improve fuel economy is to reduce the weight of the vehicle. Manufacturers are currently targeting the cooling system as an area that could stand to lose a little weight.

Most engine thermal engineers consider the coolants currently in use to be poor thermal conductors. If they could find a better heat transfer median, the size of the cooling system components could be dramatically reduced, which would lead to improvements in fuel economy.

Manufacturers and chemical companies are currently working on nano-fluids as a replacement for current coolant products. Yes, it sounds like something right out of Star Trek, but it is really just another name for a coolant that uses nano-metallic particles to reduce drag and dramatically improve the heat dissipation characteristics of the coolant.

As you can see, the subject of coolants can be a little more complex than it may appear to the average consumer. Now that you possess an understanding of coolant design and use, your customers will benefit from your knowledge on the subject.

I’d like to thank the engineers at Zerex (Valvoline) and many OEM engineers for sharing their information on this subject. Until next time, remember: Act as if what you do makes a difference, because it does.