Can I mix antifreeze colors?

Dear Car Talk

Dear Car Talk | Aug 01, 2000

Dear Tom and Ray:

I have a question about antifreeze. Many months ago, my trusted mechanic was making repairs to my 1995 Chevy Lumina. He told me he had to flush the radiator because I had added a different color antifreeze to my cooling system. He stated that one cannot mix colors of antifreeze (pink and green, in my case). Since then, he informed me that my head gaskets needed replacing. Could it be due to the little bit of pink antifreeze I added to the green? -- Jeffrey

TOM: Well, we checked with our interior decorator, Franc. And he said that under no circumstances should you ever mix pink and green. Unless you live in Florida.

RAY: From an automotive point of view, however, it's a more complicated question. The color in antifreeze is like food coloring. It's added by the manufacturer as an identifier. Prestone has always used fluorescent yellow, Texaco and Shell have always used green. So it gives you a hint as to the identity of the coolant's manufacturer, but it doesn't tell you what type of coolant it is, and that's the more important question.

TOM: There are basically two types of coolant in the world. There's "traditional" coolant, which is what we've been using forever. And then there's the newer, "organic" coolant -- also known as OAT, or Organic Acid Technology.

RAY: The difference is that the OAT coolant has rust inhibitors that last 100,000 miles or more, whereas traditional coolants have rust inhibitors that only last 50,000 miles or so. And if you mix the two types of coolant, nothing terrible is going to happen -- nothing is going to blow up or melt -- but a mixture of the two types of coolants won't have as much rust protection as either one individually.

TOM: General Motors has been using an orange-colored OAT coolant since about 1994. So that's what was probably in your car, Jeffrey. (There's usually a decal under the hood that tells you if your car came equipped with long-life coolant.) Your mechanic may have noticed that the color was diluted with something else, and he may have assumed you mixed a traditional coolant with your OAT coolant.

RAY: The experts we spoke to at SAE (the Society of Automotive Engineers) said that you can mix up to about 15 percent traditional coolant in your OAT coolant before you have any real effect on the corrosion inhibitors. So if, for example, you just top up your OAT coolant with a few cups of traditional stuff, nothing's going to happen. But if you go beyond that 15 percent threshold, then it makes sense to EVENTUALLY drain out the coolant and fill it up with all one or all the other.

TOM: It's certainly not an emergency. As we said, nothing is going to break, burn up or melt. And even if the rust-inhibition properties were compromised, you could still go a year or even several years without any negative effects. But it would be prudent, during your next service, to drain out a mixed batch of coolant and go with one type or the other.

RAY: So that's the story on coolant color, Jeffrey. Color doesn't tell you what you really need to know. You can mix two different colors of the same kind of coolant without any problem whatsoever. But if you mix a significant amount of one type in with the other type, you're weakening your corrosion inhibitors (it happened to my brother, and look at the condition he's in now).

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