Antifreeze: Red or Green? What's the Difference?

Engine Coolant Radiator Cap

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There has been a very lively discussion going on about "Red" or Dexcool® antifreeze and regular "Green" antifreeze. It's important to clear up some myths and misconceptions about both. This is quite a challenge because every company's anti-freezes have different combinations of additives and inhibitors. We won't go into brand specific formulations but rather stick to the basic properties common to all anti-freezes.


One myth is that all red anti-freezes are Dexcool®. There are standard anti-freezes that are red and cars that have Dexcool® will be labeled as such. Another myth is that Dexcool® is not glycol based. Not true, all anti-freezes are glycol based, including Dexcool®. Both ethylene glycol (EG) and propylene glycol (PG) are used as the antifreeze base. From here the additional additives and inhibitors are added. Each glycol has supporters, although the best choice depends on the intended use.


PG differs from EG in both acute and chronic toxicity. In antifreeze, we are most concerned about one-time accidental ingestion. Therefore our interest is in acute toxicity. The acute toxicity of PG, especially in humans, is substantially lower than that of EG. Propylene glycol, like alcohol, is not toxic at low levels. In applications where ingestion is a possibility, PG based antifreeze is a prudent choice. EG is the most common base used in the manufacturing of antifreeze.


Another consideration is that all anti-freezes pick up heavy metal contamination during service. When contaminated (particularly with lead) any used antifreeze can be considered hazardous. PG is not a chronic toxin. EG and heavy metals are chronic toxins. Heavy metals, on the other hand, are not acute toxins at the levels found in used antifreeze. For this reason, PG based anti-freezes are much safer for people and pets in case of accidental ingestion even after use.


In many US and Japanese antifreeze formulas, phosphate is added as a corrosion inhibitor. European vehicle manufacturers, however, recommend against the use of phosphate containing antifreeze. The following will examine the different positions on this issue to help judge the pros and cons of phosphate inhibitors.

In the US market, a phosphate inhibitor is included in many formulas to provide several important functions that help reduce automotive cooling system damage. The benefits provided by the phosphate include:

  • Protect aluminum engine components by reducing cavitation corrosion during high-speed driving.
  • Provide for corrosion protection to ferrous metals.
  • Act as a buffer to keep the antifreeze mixture alkaline.

European manufacturers feel that these benefits are achievable with inhibitors other than phosphate. Their main concerns with phosphates are the potential for solids drop out when mixed with hard water. Solids can collect on cooling system walls forming what is known as scale. The phosphate level in most US and Japanese antifreeze formulas do not generate significant solids. Furthermore, modern antifreeze formulations are designed to minimize the formation of scale. The small amount of solid formed presents no problem for cooling systems or to water pump seals.

Antifreeze: Red or Green?

While it is ethylene glycol EG) based antifreeze, the concern with mixing comes from the fact that there are very different chemical inhibitor packages in use. Most leading technologies will work very well when used as intended, typically at 50% in good quality water. If the coolants become mixed with Dexcool®, however, one study showed a possible aluminum corrosion problem in certain situations. The other question is a concern for dilution of the protection packages. At what mix is there too little of either inhibitor to protect the engine? As a precaution, both GM and Caterpillar instruct that contaminated systems must be maintained as if they contained only conventional coolant.

It is not recommended to use Dexcool® in a vehicle that did not come from the factory with Dexcool® in the cooling system. It would be very difficult, if not impossible, to flush out all the conventional anti-freeze coolant from the cooling system of an older vehicle, and any conventional anti-freeze would contaminate the Dexcool®.

Compared to old-fashioned phosphate antifreeze, Dexcool® may be more stable and improve water pump life. Evaluations of the two technologies to compare their respective service lives have found them comparable. In fact, a Ford Motor Company study concluded that organic acid coolants do not offer any significant advantages for the consumer over current North American coolants. In a modern car with a well-maintained cooling system, current North American and OEM factory fill coolant corrosion protection can be extended far beyond previous expectations.

If your car came from the factory with Dexcool®, use Dexcool® for replacement or to top off. If your car came from the factory with standard "green" antifreeze, use that for replacement or to topping off. Case in point, Dexcool® has been known to cause head gasket and water pump failure on some Ford OHC V-8's.