Several years ago, I purchased a slightly used Saturn for my wife (it was still covered by its original warranty). I thought I had done my research and that the car was pretty dependable (and time has proven it to be so). Several months after buying the car, a strange light appeared on the dash, a rectangle with a wavy line and a large arrow pointing up. Upon checking the manual, I discovered the light was telling me that the coolant in the coolant reservoir (overflow container) was low, and that the Saturn used DexCool which worked best when not mixed with any other form of coolant. After not finding it at a couple stores and then getting a concerned look and some advice from the guy at NAPA, I decided to look up DexCool on the web. What an eye-opener. I think I spent every night for two weeks reading blogs, articles, technical specs, testing results and legal briefs and I think I did a pretty good job of figuring out why DexCool got a bad rap, why GM still uses it, and what caution you need to take when using it.
DexCool is actually a very good long life coolant - so long as you don't overcook it. It does not eat seals or gaskets. It does not eat hoses. It will not hack your hotmail account. And you can mix it with other traditional green coolants. So why the bad wrap? Because people were overcooking it. The issue wasn't really with the DexCool, it was with radiator caps. I don't know when GM started using DexCool, but when they started putting it in Chevy Blazers, they ended up with a bunch of unhappy customers with lots of problems. And it started with the radiator cap. For whatever reason, GM sold a bunch of Blazers and not quite so many of a few other models with radiator caps that could not hold a seal when under pressure. Coolant escapes, engines overheat and seize... in some cases under 500 miles on a brand new vehicle. Many owners thought, "it's brand new, surely I can make it to the next gas station or to my house before something bad happens." No, this would have happened if they were running DexCool or traditional green coolant, but for quite a while, DexCool got blamed. But there were far more owners of these vehicles who were smart enough to add fluid before their engines got too hot, but the DexCool had already been overcooked. When the temperature of DexCool exceeds some temperature beyond it's operating parameters, a good amount of it boils off, but what is left turns to sludge (I'm not sure if this is an actual chemical transformation or additives that come out of solution as the liquid boils off). The sludge does not mix well with ANY coolant and can form blockages. These blockages can result in high pressures that result in blown out seals and gaskets (and the myth that DexCool eats seals and gaskets). The blockages can also result in overheating and destroying waterpumps, thermostats and other cooling system components as well as overheating that fuses your engine into a nice solid hunk of metal. So, GM recalled the radiator caps on thousands of vehicles and thought they had the problem licked. But they weren't quite there. You see, radiator caps adequate to prevent leakage of traditional green coolants weren't quite up to snuff for DexCool. I don't recall if DexCool causes the cooling system to run at a higher pressure or if the molecular makeup allows it to slip past a radiator cap designed for green coolant. But basically, there were a lot of vehicles out there losing just a little coolant every day. But over time, a little adds up to a lot and you are left with sludge. It took GM quite a while to either figure it out and/or own up to the problem. They ended up replacing a LOT of radiator caps (along with other failed engines and cooling system components). And they added additional warning lights and gauges to all there vehicles from that point forward (my wife's car has the reservoir light, a radiator light, and a water temperature gauge - and there is still plenty of coolant in the reservoir when the dummy light comes on... the first time the light came on, I thought it was still full). GM also increased the size of the coolant reservoir in all the newer vehicles (the one in the Saturn has a much larger capacity than the wiper fluid container).
You should only use DexCool in your cooling system if the vehicle came from the factory with DexCool or if the vehicle's manufacturer specifically states that DexCool can be used in the cooling system. DexCool has an incredibly long life, 100,000 miles, I think. But if mixed with any other coolant, you should change it out every 15,000 or what is specified by the other coolant, whichever is more frequent. I believe GM also says that if you drain the reservoir, you should assume that the coolant is low in the radiator and the system should be flushed and refilled with fresh coolant.
I did this research over 10 years ago, and my memory isn't what it used to be, but I think I have the gist of it right. Once DexCool started to get a bad wrap, people started blaming it for all sorts of things, most of which they really couldn't back up.
As for my wife's car, the manual said to top off the reservoir with DexCool and if we saw the indicator come on three times in three months to take the car in for service. The dealer replaced a faulty gasket (under warranty) and we haven't had a problem since. The car has well over 100,000 miles on it now. I still use DexCool in her car. I would NOT use DexCool in a motorcycle. The reservoir on a motorcycle is too small and you can drain it completely with even a small leak.
The above is based on my memory and my opinions. I am not a mechanic, chemist or automotive engineer. As always YMMV.
