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Old 12-06-2012, 03:56 PM   #9
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Join Date: Sep 2012
Location: Old Greg's Cavern
Posts: 12,618
My Ride: '13 128i, '04 330Ci
Originally Posted by OUTATIME View Post
Ok, so the mechanic did admit to me that he topped off the tank. I have searched and found nothing about expansion tanks leaking only because they are topped off. If they're not supposed to be topped off, what do you do with it?
I bet your mechanic's top off caused this. A shop I went to did the same thing, and I noticed a leak that night after taking it home. I had no leaks before. I found that they had filled the radiator to the top. Take it back to him and tell him you understand the honest mistake, but you want that leak fixed. Explain to him the increased pressures that break seals as a result of over filling. The cooling system requires an air "cushion" to build pressure and allow for the expansion of fluid as it heats.

If you heat room temperature water to near boiling, it will expand by almost 5%. To hold it at it's original volume requires quite a bit of pressure. Water will have no problem bursting seals and blowing up a composite ET to make room for itself. Water IS compressible, but it just takes a great deal of pressure to make any sort of difference.

Looking at my steam tables, in order to keep water in the same volume from 70F to 200F, you have to apply over 12,000 psi. Have fun containing that. That sounds high though I'm gonna double check on some online tables.

EDIT: Wolfram Alpha confirms. wow that's insane.

Last edited by WDE46; 12-06-2012 at 04:12 PM.
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