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Fred,
From my novice reading of the diagrams, I think you were right the first time.
It looks like the Turbo 337 up through the D model share the vent line with the return line. And it is "Y'ed" to both the aux and main. That means that the return fuel may go into either the aux or main tanks (with no actual control over it).
All other models (throughout the years) pipe the return fuel into the main tank (regardless of whether the aux or main is in use).
I have been pondering this problem since it has been posted and my best guess is that the 6 gallons present in the aux tank at fill-up time was simply the returned fuel making its way into the aux tank when John was feeding from the mains. I realize his data doesn't really support this theory.
John,
Some questions that might help to get to the bottom of this are:
How much fuel was actually present in the aux tank when the engine because starved for fuel? Doing a little bit of flight planning, would you predict that only 12 gallons were used when the engine faltered? Of course, something like a Shadin would make this really easy to determine.
When you filled the mains, did they take the amount that you would have predicted (or did they take 6 gallons less?).
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