What a PITA! We know some Mirage heat exhanger cores have already become fouled-up with gooey gunk, and we know it comes from chemical reactions from parts reacting with coolant. Expensive repairs follow. Similar problems can potentially happen inside the engine, or any other part immersed in coolant. This is entirely forseeable and preventable.
The easy, cheap and perfectly safe solution is to simply monitor the PH level of the coolant! Any chemical reaction in the coolant is mirrored by the PH value.
The PH level should be around PH 7.5 to PH 9. If PH runs off to other levels, its time to replace the coolant.
Its that simple. I check it once yearly.
BTW, all you need for testing is a pack of "Universal indicator paper" which may cost less than €2 ($2) incl. postage. A possible source is at the bottom of this post.
Below shows a test with fresh coolant. If chemical reactions change PH level, coolant should be exhanged asap! Remember, the PH scale is a logarithmic value, a small difference in number means a big difference in chemical properties.
A possible source:
Edit: The picture above was taken with fresh coolant. My coolant is now 3 years old and slowly approaching PH9. I asked the world's largest coolant maker if I needed to worry. They replied, PH9 is still OK, but it should be exhanged if it rises any more than PH9.
A word of advice:
If you want to replace the coolant, be sure to thoroughly flush the old stuff out first! Then use whatever coolant you want, provided the coolant manufacturer expressly says it is the right stuff for the car! If in doubt, contact the manufacturer before buying! If they don't expressly say its good for Mirage, buy a different product!
Flushing out the old coolant requires opening the drain plug on the bottom of the radiator, refilling with water, driving around the block, redraining, etc. Since about 40% of the coolant will always remain in the car, draining has to be repeated at several times at least. When adding new coolant, use concentrate, because there will be water remaining in the system from flushing. A "ready for use" prepared coolant would be diluted beyond usefullness.