Which Coolant to Use: DexCool or Green Coolant?
This is an issue that gets brought up frequently, especially in a GM community. I've done a bit of research on this, and put together what I found.
Which A-bodies came with DexCool from the factory?
In 1996, all Cieras and Centuries came with DexCool coolant direct from GM. There may have been some late 1995 models with DexCool, but this has not been verified.
Differences between green coolant and DexCool
There is remarkably little difference between the green antifreeze and DexCool. Both are ethylene glycol-based. Both require a 50/50 mix with distilled water. GM claims tap water is suitable for DexCool, but why introduce unnecessary minerals to your engine? The differences rest in the type of corrosion protection each product offers. Green coolant uses a silicate-based corrosion inhibitor that coats the metal. Over time, these inhibitors break down, requiring a coolant flush every 30k miles or so. In an aluminum engine, green coolant does not change color when it breaks down; so many owners have no idea when this has actually occurred until their nice expensive aluminum heads have cracked. In an iron block engine, green coolant will turn reddish-brown with rust when its corrosion inhibitors break down. DexCool (phased in by GM during the 1995 model year), on the other hand, uses an acidic (GM calls it “organic”) corrosion inhibitor that does not break down over time. However, DexCool does not coat metal components as much as green coolant does, so proper levels must be maintained to ensure corrosion protection. Because corrosion protection does not diminish over time with DexCool, it can be left in the engine much longer than green coolant. While it's disputable whether or not you should leave DexCool in for the 150k miles that GM & Texaco claim, DexCool should easily be able to stay in the engine for 75k miles, two and a half times that of the green coolant. 75k miles is generally what the average vehicle travels in 5 years, and GM does not recommend leaving DexCool in for longer than 5 years.
Finally, as a side note, DexCool, being an ethylene glycol-based mixture, is just as toxic as green coolant. I've heard people say that DexCool was created to be less toxic to animals and humans, but this is not true. 1 ounce of DexCool or green coolant can kill a dog or cat, so be careful. I think people may be getting DexCool confused with “environmentally friendly” propylene glycol coolant, such as Sierra, which is not recommended by most engine manufacturers.
DexCool: pros and cons
Because DexCool does not coat the metal, in cast iron engines when the coolant level gets low, the iron will start to rust, which mixes with the coolant and creates chunks of gel flowing through the engine. In aluminum engines, if the DexCool level gets low, the aluminum will corrode and weaken, leading to gasket failures or worse. If you properly maintain your car, you shouldn't have low coolant levels, and thus shouldn't have this problem. A lot of the horror stories we hear stem from improper maintenance (improperly mixing DexCool, letting it get low, etc..). DexCool does NOT eat gaskets, as the tale goes. Gasket failures are the result of poor maintenance, old age, or poor design.
Whether it is used in an aluminum or cast iron engine, DexCool enjoys much longer protection against corrosion compared to green coolant. It's also been found to be easier on coolant system components, such as heater cores, radiators, and water pumps. The reason you'll need to eventually change the coolant is not because of corrosion protection breaking down, but because the coolant thickens with deposits. These deposits can occur naturally from minerals within the engine, or from improper maintenance. A simple way to check and see if you need a flush is to look at the underside of your radiator filler cap. If there are brown deposits mixed with coolant stuck to it, then have it flushed out.
Green coolant: pros and cons
The traditional green coolant, which we have all become familiar with, is also very effective. However, the corrosion protection breaks down, and requires a flush every 30k miles or so. To most of us, this is not a problem at all. But without regular attention, weakened corrosion inhibitors will cause corrosion of iron and aluminum engine components. In addition, the silicates in the green coolant are abrasive and may take a higher toll on water pumps and other cooling system components. Also, if a vehicle is used occasionally, green coolant can collect in low-flow areas such as the heater core and form a gel. Mixtures denser than 50/50 can exacerbate this problem.
I have DexCool, and I'm low on coolant. But all that's available is green coolant. What do I do?
DexCool is designed to be mixed with green coolant in a pinch without much problem. There were some problems with DexCool gumming up when mixed with green coolant back in the early days, but that problem is allegedly fixed now. (I certainly hope that you are not driving around with 9-year-old coolant sludging its way through your engine. If you have a coolant-related issue and you're still running 1996-era DexCool, then quite frankly that's your fault.) Both DexCool and green coolant are ethylene glycol-based products. However, when a substantial amount of both products are mixed (although GM says it's OK), the combination of different corrosion inhibitors is believed to cause premature corrosion of components. This is because green coolant will break down the corrosion inhibitors in the DexCool. Once you are able to see a mechanic, have the system flushed and refilled with DexCool. However, since you have introduced green coolant to the system (even if it is later flushed out and refilled with DexCool), you will need to adhere to the standard 30k mile service interval until it is flushed again. After that, you should be fine going with the 75k mile interval.
So, should I convert?
Whether you are switching from DexCool to green coolant, or vice-versa, there are really too many variables to account for in order to make a solid recommendation here.
Converting to a different coolant formula always requires a system flush. Converting to from green to DexCool is no different. However, because of the coating properties of the silicate-based green coolant, it is impossible to know for sure if all of the green coolant has been flushed out. In fact, it's likely that it hasn't been completely removed without a trace left behind. Any trace of green coolant present in a DexCool system will degrade the life of the corrosion inhibitors in the coolant, drastically reducing the time between service intervals. So if you have a car that came with green coolant from the factory and want to switch to DexCool for the long service intervals, then be aware that traces of silicate coating will remain on engine components, even after a flush. It will take another 30k mile flush at the very least to rid the system of silicates. That said, DexCool shouldn't harm your engine, but it is impossible to predict how the various accessories, gaskets, etc. in your particular car will react to DexCool.
Switching from DexCool to green coolant seems to be less of an ordeal, since GM and Texaco claim that mixing green coolant with DexCool causes no damage to the engine as long as 30k mile service intervals are observed. However, there are the same uncertainties regarding accessory and gasket longevity when switching from DexCool to green coolant as vice-versa.
The bottom line is this: You can never be completely sure that all of the previous coolant has been flushed out of your engine before adding a different coolant. Whether your engine is aluminum or iron, just stick with whichever coolant came from the factory. There are too many uncertainties involved in switching coolants.
If you have green coolant: make sure you change it every 30k miles, watch the color of the coolant, and of course watch the levels.
If you have DexCool: do not let it get low, examine the consistency (make sure it's not gumming up), and change it every 75k miles
Replacing old DexCool with new DexCool = 75k mile service interval (a conservative personal recommendation)
Replacing old green coolant with new green coolant = 30k mile service interval
And if you feel that you absolutely must switch..
Switching from green coolant --> DexCool = 30k mile service interval for the first time, 75k mile interval thereafter.
Switching from DexCool --> green coolant = 30k mile service interval