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Hello all!
Regarding the '67 T and I assume John's '69 T: Excess fuel from the engine driven pump (fuel not consumed by the engine) returns to the manifold from which the pump is simultaneously drawing more fuel than it is returning. The difference is made up from the selected tank. Return fuel does not go up the vapor line and into the tank. The vapor line comes off the very top of the manifold allowing any vapor to work it's way into the vent system. The vapor line off the manifold has no check valve nor any means of separating fuel from vapor. As the manifold is located well below the level of the tanks, head pressure from the selected tank will force fuel up the manifold vapor line to equal the level of the top of the fuel of the selected tank. However, the vent lines are above the highest fuel level, so when everything is right, no fuel should return to a tank. Since the main is slightly higher than the aux, head pressure from a full main would send liquid fuel a little higher up the vapor line but not enough to reach the tee and get into the aux.
One good reason to burn the aux early in the flight with this system is on a long flight over hostile or wet terrain, like Galveston to Belize City, should an engine failure occur, the fuel in the aux tank is not available for crossfeeding.
None of this answers the question of why John's engine quit and why there was 6 gallons in the aux tank. John, have you had a chance to go out to the plane and do any investigating?
Bawb
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