Main tanks leaking into Auxs on C340

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      Good Morning All,
      I just noticed that while sitting for a few days the Main Tanks were almost empty I switched to Aux gauge and they were almost full. (right side)
      I am assuming that the problem is in the Main/Aux switch on the floor ?
      Any thoughts would be appreciated.


        The easiest thing to try is to cycle the valves a few times. If you had dirty fuel, then maybe this will reseat the balls in the valves. Of course, you will need to burn off the aux fuel and put some fuel in the mains first. It is a real tough job to remove the fuel selector valves so try the easy stuff first. Besides the selector valves are not field fixable and I believe the cost is $2 K plus 3 to 4 weeks downtime.


        Both sides?

        I’ve had one aux tank “spew” fuel when I popped the cap on pre-flight. Main tank was down a bit. Turned out to be a check valve in the system.


          No..only right side


          First, easiest, (cheapest), is the check valve, I think. I suggest you fly it again. As Dick Welsh says, it may be just a “speck” of dirt headed for the sump. That’s what mine was… Local FBO has replaced check valves in Part 135 340’s.


            Thanks all for the thoughts…..
            I especially like and will try the idea that there may just be a little dirt in the valve.
            I used to have a Twin Comanche that developed this problem but they did not have check valves.
            I will do some flying and see if that helps, check valve next.


            Be sure to position the floor selector switch to off at every shutdown. Your problem may be in the selector valve which is out in the wing. These valves are expensive to repair. Sometimes only an o ring needs to be replaced. There are several posts in this maintenance section about these valves. Several different valves were used. Good luck. Glenn Carwell 1981 340a


              Thanks for all your suggestions.
              As it turned out i did not have to do anything.
              After draining each drain and changing the selector a few times it appears I no longer have a problem.
              Great way to fix a problem …………no $$$$$$$$$$$$$$$$$


              There are two brand of valves. Both work similar. Each inlet bore has a nylon/phenolic bushing with an O-ring backed by a spring.
              Each bushing presses against the swash plate which has holes at various places to pass the fuel.

              My opinion is sometimes the bushings can gets cocked sideways a little bit and stickin the bore.

              These valves are not difficult to take about for celaning and fresh O-rings. Be very careful during disassembly because there will be small check balls in various places that will fall out.

              I clean and polish the bores with 2000 grit wet sandpaper.

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