Main Fuel Seeps to Aux Fuel

[air2mud]

My 1967 O-2A is letting fuel from the left mains into the left aux tank. I've wiggled the selector valve a bunch with no success in stopping it. It's not venting any fuel overboard and my A&P is all jammed up. Any problems flying it until I can get it into the shop? I'm able to use all the fuel in the left wing but my concern is that may stop sometime.

[SteveG]

I too have experienced a similar issue but in the right wing of an "A" model. If I or a lineman manages to fill the right main to overflowing it sets up a siphon effect in which fuel begins to flow from the main into the aux tank. This becomes a problem when the aux tank becomes full and starts to drip fuel from the vent tube. Drawing a quart or two from the aux tank through the sump drain breaks the siphon at which time I merely return the drained fuel back to the main tank and all is well until the next time the main is overfilled. An examination of the fuel schematic would suggest that this is not possible but it most assuredly happens. I presume that there is some minor deficiency in the fuel selector valve which allows this to occur but trying to overhaul the selector valve to prevent it from ever happening again does not seem worth the expense incurred to preclude a once a year or so event. In all other respects the fuel system behaves normally. I have not experienced any anomaly in function if one just uses care in fueling. I have never noticed this phenomenon during an en-route fuel stop when the engines are immediately restarted but it can happen when the tanks are overfilled and the aircraft is then parked and allowed to sit.

[air2mud]

Thanks for the heads up. After what you said and talking to my A&P it looks like flying it will not be a problem if I manage the fuel correctly. We're planning on fixing it at annual. MERRY CHRISTMAS!!