http://volvospeed.com/Repair/thermect.htm

Here's some info on coolant:

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.

--
Jack