I would flush the coolant and put in G12 or the newer G12+. Here's a cut and paste of the basic idea of why (thanks google):
"O.K., now my opinion about this G11 to G12 changeover. In order to
understand this better, I'll tell you what I know about coolant. Buy
only high quality antifreeze-cheap brands can be straight ethylene
glycol minus important corrosion inhibitors and lubricants. ALWAYS mix
50/50 with water(preferably distilled) or follow ratio recommend for
your climate. Never top off coolant tank with straight
coolant-preferrably small amount of distilled water or your 50/50
mixture. Use common sense, large amount missing means that if you
refill, you are going to throw off the glycol to water ratio, and it is
very important. Antifreeze should never exceed 65%. Exceeding 85% will
cause the silicates to drop out of suspension and goo up to clog the
radiator and reduce heat transfer. VW recommends the water and
compressed air treatment to upgrade to G12 because up to a third of the
coolant is still trapped in the heater core and the engine after you
pull a hose or the drain cock. This flush ensures that you are removing
all accumulated rust, scale,silicate buildup and old coolant as best as
possible. By the way, the blue and red coolant will foam up and turn
brown in your expansion tank if you mix them or don't get all the G11
out. The degree of corrosion that takes place in your VW depends upon
the type of minerals and alloys in the engine and radiator, and the
acidity or alkalinity of the coolant. So long as your coolant remains
alkaline, corrosion will be held to a minimum. Conversely, acidic
coolant hastens the corrosion process that occurs between the cast iron
and the aluminum present in the engine and radiator. The corrosion
inhibiting chemicals that are added to you coolant is what keeps the
alkalinity on the high side of the Ph scale. That's why adding
aftermarket wetters and boosters is not smart because you are altering
the already unknown alkalinity of your coolant(no matter how new, it
varies depending on mix ratio, mineral content, additive content) More
important, this alkalinity ratio doesn't have to be bigger to be
better-it just needs staying power. This is measured as alkalinity
reserve(how long your coolant can resist corrosion) The enemies of your
coolant are heat, dissolved oxygen and minerals which react with the
metal surfaces in your engine depleting the capacity of the coolant to
resist becoming acidic. Therefore, changing the coolant annually or at
least bi-annually guarantees that you never exceed the coolants ability
to resist corrosion. European car makers like VW specify coolant
additives lacking in phosphates and including borates and low silicates
because their water is harder and it reacts with phosphates to create
calcium and magnesium deposits. The Japanese disagree and specify high
in phosphates and low in borates and silicates because they fear lack of
maintenance will cause borate corrosion. This is the reason you see the
little "phosphate free coolant " only from vw under the expansion tank
cap-or it will void mf's warranty. It seems that in my opinion, the
original G11 coolant must have been a poor acidic retardant-either from
the reaction to the water installed from the factory and/or an additive
package that was insufficient to go more than a couple of years on North
American water. Mixing the two coolants causes problems because you get
saturation of the silicates and gelling inside the engine, thus the
specific ritual flush with water and compressed air. I firmly believe
that no matter what proportion VW used for an additives package with the
G12 coolant, it too should be flushed out and refilled every year or
two. Considering that it isn't that much work and it beats playing with
litmus paper and rebuilding your cylinder head. By the way, the
corrosion we usually see on VR6s is surface etching that rubs away to
leave pits-almost always around any flange that attaches to the head, or
on the water jack inlets comprising the head gasket. They're usually
good to go because they are far enough away from critical sealing areas,
but we do replace heads when they are questionable. I really doubt that
the new metal gasket is the reason for the G12. There is actually no
bare metal on the gasket and all the passages have sealer from the
factory around them. I think the reason was simply that G11 didn't
provide long enough reserve capacity and coupled with poor maintenance,
they were getting too many warranty problems."
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Originally Posted by budfox
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