Fuel Tanks Cross Feed

[TheFlyingScott]

Hi Guys

I recently purchased a 1976 M20F pretty much in stock condition. It had sat without flying for 5 years. I did pre-purchase inspection and everything checked out pretty much as expected. My problem is, it appears my left fuel tank is cross feeding into my right tank! I know this should be impossible since there is no direct connection between the two tanks except through the fuel selector valve. The fuel selector valve is suppose to connect one tank at a time to the engine. Last time i filled both tanks I left about an inch below the rim to allow for expansion. When I came back the next day during the preflight walk around when I inspected the tanks visually I found the left tank down a couple of inches and the right tank full to top and leaking fuel from the vent line. I guess my question is, is it possible that the fuel selector valve is not sealing properly and allowing fuel to flow from one tank to the other? 

I set the fuel selector valve to OFF this time to see if the problem persists (I haven't checked yet).

 

Michelle

N6987V

 

[Jeev]

Just a shot in the dark but was your Mooney parked outside with right wing (full tank) in the sun and left in the shade?  If the air temp went down since fueling and only your right wing tank was exposed to the sun this could happen.  Otherwise I would take a close look at the fuel selector which you should do anyway if it has been sitting for 5 years.  Good luck

[Guest]

It does not really make sense for fuel to transfer, if both were at the same level and the plane was level. Heating as mentioned would be a possibility.

Clarence

[TheFlyingScott]

It was parked outside but both wings got equal amount of sun. My suspicion is that the rubber/silicon seals (if there are any) in the fuel selector are failing and need to be replaced. No signs of fuel leakage under where fuel valve is, no signs of leakage under wings where fuel tanks are.

[mpg]

It was parked outside but both wings got equal amount of sun. My suspicion is that the rubber/silicon seals (if there are any) in the fuel selector are failing and need to be replaced. No signs of fuel leakage under where fuel valve is, no signs of leakage under wings where fuel tanks are.

Well,,,   It seems obvious to me also,,  that fuel is getting by the selector,,,   but

were you parked on the side of a hill?

[Hank]

My guess would be that the left tank is leaking and needs to be patched or stripped and resealed, that's why it was low. The right tank got hot, expanded and dripped out the overflow.

Have someone knowledgable check the left tank. This is best done empty, so fly as much fuel out as you are comfortable with. Often there will be little or no visible staining during warm weather, as the leaking fuel evaporates quickly. In cold weather, these small leaks rapidly become worse, and due to slow evaporation, blue stains appear on the wing and the ground.

Another possibility would be the gaskets around the fuel sender in the wing root. I would be beyond shocked if fuel was leaking uphill from the fuel selector into the right wing--what force would push the fuel to make the level in one tank higher than the other?

Hope for a fuel sender gasket leak. Spot seals in the tanks run around $1000, while a strip and seal will run almost four times that per tank. Gasket leaks are much less expensive.

[carusoam]

Moonies are so efficient they appear to make their own fuel...

1) This is a new challenge for me. Never heard of this one...

2) Selector valve issues have never been good. Sticky or stuck selectors...

3) I learned early on, there is no both position. My first Mooney taxi...

4) There isn't a way (that I know) for fuel to transfer from one tank to the other, intentionally.

5) Water has made a few problems for Mooney pilots...

6) worn or leaky selector valves usually run out to the ground via the belly panel...

Please take this advice as friendly push to find the source of what you observed.

Some fuel problems can be very serious.

I am only a PP, not a mechanic...

Welcome aboard,

-a-

[N201MKTurbo]

I can't imagine any failure of the fuel selector that would allow more then a slight trickle of fuel to cross feed. It would take weeks for anything noticeable to happen.

[Steve65E-NC]

Might be worth checking your tank vents.  If one is plugged it might cause enough heat or cold induced vac or pressure to pull fuel even through good seals.  If I have this right, I should get a prize.

[Bob_Belville]

Even if there was an open shunt at the fuel selector valve if the vents are open the two tanks would seek the same level, not 2" difference. 100LL obeys the same law of gravity as H2O. Unless the ramp is pretty not level. If it isn't, is there more fuel in the lower wing?

[gsxrpilot]

Whatever it is, it won't be cross feed through the fuel selector valve. It's uphill both directions. A leaky tank is a sure possibility and sitting out in the sun, it will evaporate before you can spot the leak.

[Alhall]

I suspect one tank expanded and the other siphoned through the vent or the cap with a hose.

[Bob - S50]

I'll throw in a couple more possibilities.

 

For the low tank.  When you fill it that close to the top, the inboard portion should be full to the top.  However, there are ribs in the wing.  I believe there should be little 'vents' in the ribs that allow fuel and air to pass the rib.  If those little holes are plugged, you'll end up with trapped air in the top of the inboard portion of the wing.  If these holes are still there but very restricted, the wing could appear to be full when you put the cap on.  Then, over a period of time (minutes? hours?) the fuel finally displaces the air and your apparent fuel level has dropped.  This is your true fuel level.

 

I don't know the F, but if you have fuel tabs at 50 gallons (25/side), try filling it to the tabs, then see how much more it takes to fill the wing.  If you have 64 gallon capacity and you can't fit another 7 gallons in, I would suspect trapped air.

 

For the high wing, as someone else suggested, did it rain?  If so, did you sump the tank to see if there was a bunch of water in it?  Could be the cap was not sealed properly allowing water to enter the tank.  Then as the fuel expanded from heat, it began to vent and started a siphon.

 

Good luck,

 

Bob

[TheFlyingScott]

With the amount of fuel missing from the left tank (gallons) I would imagine it would leave a telltale stain if it was a leak from the tank or the fuel selector, as I said there is no indication at all of a leak, no weep, no seep, no stain in wheel well, nothing on top of wing, nothing on bottom of wing. There is a noticeable stain on the tarmac under the right tank fuel vent but nothing under the left tank fuel vent. So I'm going to go on the assumption that the left tank vent is plugged causing the left tank to pressurize and force fuel through the fuel selector into the right tank. Good news is, no fuel tank leaks, of course the blocked vent is not good, I'll check it out when I get out to the airfield shortly.

 

Thanks for the input guys.

[Bob_Belville]

With the amount of fuel missing from the left tank (gallons) I would imagine it would leave a telltale stain if it was a leak from the tank or the fuel selector, as I said there is no indication at all of a leak, no weep, no seep, no stain in wheel well, nothing on top of wing, nothing on bottom of wing. There is a noticeable stain on the tarmac under the right tank fuel vent but nothing under the left tank fuel vent. So I'm going to go on the assumption that the left tank vent is plugged causing the left tank to pressurize and force fuel through the fuel selector into the right tank. Good news is, no fuel tank leaks, of course the blocked vent is not good, I'll check it out when I get out to the airfield shortly.

 

Thanks for the input guys.

I am very skeptical than fuel can feed from one tank to the other through the tank selector valve. Please post what you find. 

[bonal]

First off I don't think any one said welcome and congrats on your Mooney, I cant think of a way fuel would transfer and even though Carusoam makes a very good point on how efficient Mooney's are I am confident its not making fuel so I cant answer to the increase in the one tank. As for the one that lost fuel and if you park outside my paranoid nature would first think someone stole some fuel I know its happened to me when I used to park outside.

any way welcome and I hope you find the solution old airplanes can be quite perplexing because of all things mechanical these things really do have souls.

[carusoam]

Welcome aboard, again (last line of my post above

[TheFlyingScott]

Well I checked the fuel tank vent line and found no blockage, at least as far back as I could push the wire. But the fuel transfer has stopped since I set the fuel selector to the OFF position.