So this is weird; I think I found what the issue was. The pump was leaking between the outer gasket and the block surface. This was true even with Dan's (best in my opinion) copper spray method. The surfaces are true/flat, no warpage on the pump body or block surface, it's essentially not forming a an effective seal between the outer gasket and the block surface. The gasket between the pump body and plate sealed and the outer gasket plate seal was also good, no leaks there on pressure tests.

I was discussing this with my Porsche buddy and he said the Porsche folks typically take a fine grit sandpaper to the block surface because it gets glazed and won't make a seal. I'd never heard of this issue in a water pump context.

Sure enough, I did a close inspection of the block surface where the water pump mounts. It indeed was so clean and shiny it actually felt "slippery" to me, like there was a glaze or Teflon coat on there, some film maybe. I'm not sure how this came about, but I tried a bit of 220 grit on a small spot and it had a hard time penetrating, i.e. I couldn't produce a non-slick spot with rubbing. It feels like when you cure an old skillet by heating grease in it, that sort of surface. This is why it was still leaking, the copper coat did a better job than the RTV, but neither really made a good bond with the metal, air could still pass through there. The copper spray did way better because it stays tacky. There were some areas where both stuck pretty good and I had to razor it off, but mostly not.

I'm sure I can get that slick surface off there, but it is really smooth, like glass smooth and although dry, it feels slick somehow. This is true even after wiping with solvent (Chemtool) and taking a fine wire brush to it. It's like I polished it somehow...the problems of a clean freak.

FYI,

Brian

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