Not sure if this is the post you were talking about but I'll post it anyway.

I have tried to imagine a few common questions that t you might ask about coolants if we were trapped in a British pub and your car was leaking coolant outside.

How long does coolant last?

I have read bench test results that evaluate stabilized phosphate-containing coolants in which silicate concentration was stable for more than 700 hours. Aluminum corrosion in these tests was virtually negligible. 15,000 miles per year is about 500 to 700 hours, so you can definitely trust your coolant, phosphate containing or not, for about one year of average use. The coolant will probably last even longer than that, but I have not seen test results that indicate exactly how long silicates remained stable in this coolant. Given that in service time is much harder on the system then storage time, coolant in a low use car would probably last for nearly 700 hours of engine use (accounting for cool down time), regardless of the total elapsed time. Keep in mind, however, that coolant has a definite lifespan even in storage. It would be wise to replace the coolant in an aluminum engine car at least every 2 or 3 years, no matter how little used the automobile is.

How can coolant go bad?

The primary failure of coolant is depletion of some inhibitor. For and aluminum block automobile this is the silicate. Another failure is saturation with some metal salt, such as aluminum phosphate, which then precipitates at some inconvenient location in your engine.

Should I use tap water or distilled water?

I recommend distilled water. If you have exceptionally soft water it would be acceptable, as well. Basically, the engine metals are going to corrode to some extent no matter what water you use, and hard water will encourage the resulting metal salt to precipitate.

How often should I change my coolant?

I recommend that you flush and refill annually with 50% coolant-water mixture. I expect that if you do this you will never experience any cooling system corrosion problems in you car. In storage a good coolant will probably last for 3-4 years in a TR7, which has an iron block and aluminum head, less in an aluminum block car, like the TR8, and probably longer in an all iron engine.

What coolant should I use?

After my research, I will suggest two possible choices. If you are willing to religiously flush and change your coolant annually, any aluminum compatible coolant will do. I have a strong suspicion that "aluminum compatible" are the code words for "Contains Silicates". As long as a decent silicate concentration is maintained, the presence of phosphate is unimportant. The bench test mentioned above showed that aluminum corrosion was extremely low in the presence of phosphate, as long as the silicates were not depleted. By the way of comparison, in control test with unstabilized silicates the aluminum corrosion rapidly accelerated one the silicate concentration dropped. If you don't think you are up to annual changes, I would recommend a phosphate-free aluminum compatible coolant. If you use your car with any frequency, you really should never go more than 2 years without changing the coolant.

Can I use propylene glycol?

I think that this is a matter of personal preference. PG is still more expensive the EG, but it is definitely less toxic or environmentally damaging. The performance of inhibited PG coolants is comparable, indeed better, than EG in most cases. The only negative effect I know of is that inhibited PG is less effective at preventing cavitation pitting in steel than inhibited EG.

Why is coolant green?

Because SAE Handbook says it's supposed to be dyed green or blue-green. By using standard colors we can tell what fluid our Triumphs are currently leaking by simply examining the stains on the ground.

What about corrosion on the outside of the radiator?

Although not related to the coolant, I thought I might mention that brass-copper radiators tend to lose about 5% of their heat transfer capacity over their first couple of years. Apparently this is caused by corrosion at the joint between the tubes and fins in the radiator. Painting the radiators helps prevents this corrosion. Using matte black paint may also enhance heat transfer rates. Of course, putting on excessive paint will insulate the radiator and lower its cooling efficiency.

"What fool wrote this?" or credits

The information herein is as accurate as possible and consistent with my experience and training (B.S.,Ph.D.) in chemical engineering . Reproduction in part or whole is permitted, on the condition that these credits are maintained

Gregory T. Fieldson