With so many different coolants out there, it's important-make that essential-to know what's safe to put in where, and when.

Lift the hood of a new Ford vehicle and you're likely to see a yellow coolant in the overflow jug and an interesting label on it. In pictorial language it says "Do not use orange coolant; yellow coolant is okay."

Sounds pretty straightforward, right? It isn't. Lift the hood of another Ford product (in this case, a Taurus with the pushrod V6) and you'll see that same label, but the jug contains orange coolant. Wait a minute. Something clearly is very wrong. It cautions "don't use orange," but the factory-installed coolant is orange.

Lift the hood of a Chrysler product and you'll see orange coolant in the jug and a "special engine coolant only" warning on the cap. Isn't DexCool the special coolant, and isn't it orange? Yes to both questions, but Chrysler Group says don't use DexCool in its products. It's confusing, to say the least.

The basic answers to what coolant to use where, and when, are pretty simple, but when you go past that, you're getting into some pretty complex territory. And you have to know what coolant you're dealing with to be sure you're doing no harm.

Longtime MOTOR readers know that the color of the coolant dye really is meaningless. Dye should help you spot a leak, but that's about it. But with these seemingly contradictory warning labels, you really need a basic understanding of what's been happening with coolant formulations and colorings.

You may remember that about 93% of most coolant is ethylene glycol, another few percentage points are water and/or a solvent to keep rust/corrosion inhibitors in solution and the remainder are those inhibitors. The inhibitors make a huge difference, and they're what all the arguments are about.

Didn't we talk about all these coolants last year? You bet, and we'll probably be talking about them for years to come. Since last year, however, Honda and Toyota have moved strongly into extended-life organic acid technology (OAT) coolants. Yes, DexCools also are OATs, but these Japanese formulas are not DexCool, and the two car companies have indicated they absolutely, positively don't want DexCool-type coolants used in their vehicles.

Nevertheless, you have to pick something to install, and to top up with, and we've learned that the systems are not necessarily forgiving of some mixtures of different coolants. There are circumstances where an unfavorable mixture can cause an increase in corrosion.

Taking these issues a step further, this year we've seen more cases that contribute to coolant confusion. These days, it's all too easy to make a mistake.

You have to appreciate a bit of chemistry about the various formulas. You've got to know in basic terms what's different about the different coolants-both conventional and extended-life types-and what it means when you have to pick one. Yes, that includes a bit about the dye colors.

What's In DexCool?

You should know that the term "orange coolant" has come to mean a DexCool-approved brand but that doesn't mean it's really true. If you make this assumption, you'd be wrong. It's what Fords warning labels could be interpreted to refer to, so that label doesn't serve to clarify things (certainly not when a Ford system contains a very different-non-DexCool, but orange-dyed-coolant). The discontinued Mercury Cougar was an exception; it did contain an orange coolant similar to DexCool.

The "DexCool" designation means the coolant passes General Motors performance testing. Although DexCool is not a specific formula, all three brands that have the label (Texaco Havoline, Prestone Extended Life and Zerex Extended Life) are somewhat similar. In particular, they're OAT coolants, but the similarities go beyond that basic description.

All DexCool-approved coolants to date use two organic acid rust/corrosion inhibitors, one called sebacate, the other called 2-EHA (which stands for 2-ethylhexanoic acid). These organic acids are very stable and last a long time, although they take thousands of miles to become fully effective in protecting coolant passages.

GM recommends a DexCool change every five years or 150,000 miles, whichever comes first. Because most people drive 15,000 to 20,000 miles a year, that translates to a five-year replacement interval. As noted, the thousands of miles required to protect metal is an important trade-off for that longer life. Although like conventional coolants, OATs also contain other inhibitors, for targeted protection.

The inhibitor 2-EHA works well in hard water and is more effective than sebacate at lower pH levels (when the coolant moves from the alkaline end toward the acid side), particularly for cast iron. Well, GM has a number of cast-iron engines. When there's a low coolant level in the coolant passages, the exposed cast iron rusts. Apparently, that rust is washed away later by flowing coolant, and is deposited in the heat exchangers. It eventually produces the rust powder problems that have been so widely observed (see MOTOR's August 2002 issue at www.motor.com). Why does the coolant level in these engines drop? The original radiator cap design was blamed for some of the issue, but there probably are a number of causes, including owner neglect and normal seepage. However, the rust powder issue is not a problem that was observed with the previously used conventional American coolant.