Thanks Larry,

I am still paranoid about causing more damage. I still have the old
cap and will test it; the mechanic can't find anything wrong - no
hydrocarbons in the coolant from a combustion leak, etc.

Still surprised about the TWO differential coolers, as they are metal
- wouldn't a ton of plastic and rubber parts blow first? Maybe this
was a separate problem related to the transmission???

I have been very lucky - over 600,000 miles of VW driving before my
first transmission problem.

Guess we should all test our caps on a regular basis!!!!

>  >
>>  >Paul Cavanaugh wrote:
>>  >
>>  >>  Well, the mystery continues...
>>  >>
>>  >>  My mechanic tested the coolant (while the engine was both hot and
>>  >>  cold) for traces of combustion leakage (hydrocarbons in the cooling
>>  >>  system) and found nothing.
>>  >>
>>  >>  He also found no excessive pressure in the oooling system (I replaced
>>  >>  the expansion tank pressure cap 2 weeks ago, just to be safe).
>
>I am quite certain that your problem was caused by a bad pressure cap that was
>not releasing when it should.  With a working pressure cap, the pressure
>should never build up high enough to cause the repeated damage you describe.
>   It is logical that there is no excessive pressure now if you have recently
>replaced the cap.
>
>Larry A.