- Will adding this little amount to a 50/50 mixture already in the car do any harm? If so should I add some water to the reservoir?
If the cooling system is at a true 50/50, adding a little more coolant will not cause an issue. 50/50 is a good mix, but the system should do just fine up to a 70% solution of antifreeze over water. In the future, using from a premix bottle of 50/50 will preclude this issue.
- What type of water should be added, distilled or de-ionized? I’ve read very conflicting info about each with lots of people confusing the two for the same thing?
Distilled should be used. De-ionized water will still have impurities in it, which is what you want to avoid. Again, using coolant out of a premix jug will save you the trouble.
- Does pure 100% coolant stored in an opened but tightly capped container last forever? I’ve had one for about 10 years that I still use. Color looks the same since day one
It is my understanding, as long as the jug is closed up, it's usable. When you turn antifreeze into the recycling place, they basically just filter it through really well, reseal it in a container, and put it back out for sale. In fact this website states:
Is used antifreeze as good as new antifreeze?
In a word: yes. The American Society for Testing and Materials and the Society of Automotive Engineers have done repeated studies on recycled antifreeze and determined it performs just as well as the new product. In fact, the EPA posits that secondhand antifreeze is actually better because the recycling process removes chloride often found in new antifreeze.
Why would that be important to this conversation? Because if the antifreeze doesn't have any of the impurities you'd find through use, it should still be good because there's nothing there to make it bad. A closed container should keep antifreeze in good shape indefinitely.
