Prestone Antifreeze question for 1990 200tq

[Scott Fisher]

--- jmcelroy <jmcelroy at metrocast.net> wrote:
> My 200tq looks like it has green Prestone Antifreeze
> in it. I called the Audi specialist that worked on
it
> for the previous owner, and they say that just about

> all antifreeze now days is compatible with aluminum
> in cooling systems.

That much is true.  I use fairly generic coolant (and
distilled water -- it's what they ask for) in the
Alfas, which have aluminium blocks and steel liners
(wet sleeved liners) plus brass radiator header tanks.
 (The 356 just turns up its nose at the whole
discussion of radiators and coolant and wonders how a
grandson and nephew could have gone so wrong...:-)

> Any thoughs on this subject?

There's "compatible with aluminum" and there's
"phosphate-free."  Audi specifies the latter, and I've
assumed there's a difference, so I have always put
phosphate-free coolant in my Audis.  I'd love to know
(at a chemical level) why -- what specifically goes on
with phosphates in an Audi engine that makes it a Bad
Thing?

My very-partially-educated guess involves an
understanding of the negative effects of even fairly
mild acids on aluminum -- you're not even supposed to
cook tomato-based sauces in bare aluminum pans, for
instance.  So I've always guessed that some chemical
reaction with phosphate-based antifreeze under heat
and pressure can result in the formation of phosphoric
acid, which would attack the aluminum fairly rapidly.

But as I say, that's only a very partially educated
guess.  Who's our resident chemist again -- Bob Myers?

Unfortunately, one thing I *can* verify: if you use
very hard, iron-rich water in an Audi V6 for about a
year after it develops a slow head-gasket leak, you
can lose your water pump due to lack of lubrication
and the presence of a reddish-colored sludge in your
cooling system.  Even if you don't warp the heads, the
repair costs more than most of the "interesting" cars
I've owned.

--Scott Fisher
  Tualatin, Oregon (red iron-rich soil)



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