On Mon, 11 Sep 95 10:55:38 CDT Mr.Roger Bowman said:
>I really don't understand the need to keep the phosphates out of the system,

this is what i dug up a while back on the subject:

COOLANT/ANTI-FREEZE:
   Recommended coolant/anti-freeze:
      Autobahn ZVW-237-104
        antifreeze & summer coolant
        Phosphate free formula for use in
        Volkswagen & Audi water cooled vehicles
      Ethylene Glycol based, Phosphate free
        recommended mixture:
           not less than 40 percent coolant, not more than 60 percent.
      sold at VW dealers only.
      manufactured by BASF  Questions? Call 1-800-669-2273
      BASF Customer Service:  1-800-445-4134
      BASF Marketing Service: 1-800-367-9865
      BASF Technical Service: 1-800-521-9100
      Info from Marketing Service:    <as of 03/94>
        BASF makes a 'generic' brand of GUARANTEED Phosphate Free
        coolant called ZEREX EXTREME 450.  It has been approved by
        VW, BMW, MB, Audi, Saab, and Volvo.  It is also GUARANTEED for
        four (4) years and 50,000 miles.  They said you CANNOT mix this
        with any other coolant; you must flush the system and fill with
        the new coolant/water mix.
      Info from Tech Svc:              <as of 03/94>
        BASF makes two (2) european formula coolants.  One is sold by
        VW, Audi, and Mercedes Dealers.  The other is sold by BMW, Saab,
        and Volvo dealers.  The reason for the phosphate free formula
        is basically two-fold:
        (1) Cosmetics: Hard water reacts with phosphates in coolant to
            form scale deposits in overflow bottles.  These deposits
            could form in other parts of the cooling system, causing
            blockage and overheating (see next part).
        (2) Corrosion: Phosphates tend to aggravate any corrosion that
            might already be present with any aluminum parts in the
            cooling system.  Aluminum Phosphate is formed, which is
            insoluble in the ethylene glycol solution of the coolant.
            The aluminum phosphate will tend to settle out in the cooler
            areas of the cooling system, causing blockage.  This can
            cause overheating of the engine, which will accelerate the
            aluminum corrosion.  In other words, the corrosion will tend
            to feed on itself until damage is done to the engine or the
            coolant is replaced (refreshing the inhibitors).

        Engine coolant should be REPLACED every two (2) years to refresh
        the silicates and other corrosion inhibitors contained in the
        coolant.

        He was not aware of any problems with coolants containing
        phosphates, but did say that if the coolant was NOT replaced
        regularly, the inhibitors would lose their effectiveness and
        become weak.  And that weakness could allow some aluminum
        corrosion to start, and become aggravated over time.

and from personal experience, the Mercedes folks are fanatic about the
phosphate-free stuff being the ONLY thing to use in their engines (the
service folks, that is). and i know of one toyota truck that
lost a radiator and head gasket because of using a coolant that contained
phosphates (in contradiction to the requirement by toyota that only the
phosphate-free coolant be used).
joel