Fuel Transfers from Right to Left overnight.

[skippy2]

Now if we can figure out a good experiment for my fuel tank dilemma!!
I came back from the annual about a month ago with ~3/4 in each tank. One week ago I went out to check on the plane and I was bone dry (on the gauge) on the right side and at no take off on the left. I figured someone had siphoned them dry. The next week I put 13 gals in the right tank and the gauge read ~3/4+ with the left at no take off. The next morning when I did the preflight the right tank indicated no take off and the left had 3/4-. I flew for an hour on the left tank and it was down to ~1/4+ with the right still at top of no take off.
Question - any ideas on how this may have occurred? During the preflight the right tank had ~1 inch from the bottom with fuel when I had put 13 gals in the night before. The left tank had more than the day before????
Any ways to experiment?

[GAHorn]

One possibility is an early fuel selector valve or a later one with leaking check valves. With the selector in the BOTH position overnight, fuel might trtansfer from one side to the other...and even continue if the airplane is not sitting perfectly level. After one wing is slightly heavier...the flow continues. (Cessna considers airplanes to be "level" if the distance from the ground to one wingtip is within 3" of the distance on the other side. This can lead to some considerable fuel transfer if the above situation exists.)
Some owners store their airplanes with the fuel valve in LEFT or RIGHT. I recommend everyone store their airplanes in the OFF position, not only for this reason but for safety reasons. A broken line, leaking hose, or failed fitting can flood a hangar full of planes or the ground beneath yours with 40 gallons of fuel.

If your tanks don't appear to feed equally in flight, there's a completely different situation. It's likely that your tanks are not vented equally, and although both tanks actually do feed the engine equally, one of them may transfer fuel to the other tank simultaneouisly. (The fear that one tank won't feed can be disproven by selective use of the valve.)
Full tanks can siphon fuel from the opposite tank across the over-cabin common vent line. This will make it appear that one tank is the only one feeding the engine, but it's just that one tank is replenishing itself from the other tank simultaneously. When the one tank reaches a sufficiently low level, the "full" tank will stop siphoning and give up it's fuel.

[Robert Eilers]

I have experienced the siphoning from one tank to another through the over head cross vent when filling my 170. I religiously stick my tanks before filling them. I often find the first tank I fill takes more fuel than I expected and the second tank I fill typically takes less fuel than I expected. After some head scratching I realized fuel was siphoning from one side to the other through the cross vent.

[skippy2]

Well the siphoning issue had me boggled but with the potential leaking check valves that would make sense. Wouldn't both check valves have to be leaking to allow fuel through the selector valve?
That Cessna leveling technique doesn't make sense because the ground could be sloped but both tips would be an equal distance to the ground considering the weight is centered.

[skippy2]

P.S. Thanks for the reply!

[GAHorn]

Well the siphoning issue had me boggled but with the potential leaking check valves that would make sense. Wouldn't both check valves have to be leaking to allow fuel through the selector valve?
That Cessna leveling technique doesn't make sense because the ground could be sloped but both tips would be an equal distance to the ground considering the weight is centered.

Cessna is not responsible for curvature of the earth....only the specification that both wingtips should be equidistant from the earth ....(+ or - 3".) This measurement may not be taken with one wing over a drainage ditch.

Seriously, the airplane should be leveled by using shims (flat washers) beneath the inboard end of the landing gear leg at the extrusion within the gearbox. On level ground/hangar-floor the difference between wingtips should be + or - 3". In a perfect world, a carpenter's level, laid across the seat tracks should indicate level....when the tailwheel is resting on a table or jack....with the upper door-sills also level fore-and-aft (using a carpenter's level.) AND....this is the position wt and bal computations should be made (aircraft weighing.)

[blueldr]

I don't remember any check valves associated with the fuel selector valve in the later style valves.
I seem to remember it as: Both= both tank inlets and engine port open.
Left = left tank and engine port open. right
tank closed.
Right= right tank and engone port open. left
tank closed.
OFF =right and left tank open, engin port
closed

Fuel will cross transfer via the selector valve in the off position.

Correct me if I'm wrong.

[tshort]

Mine does a similar thing. In flight, if both is selected, it will drain the L tank to nearly empty before burning off the R tank(s). I think it is siphoning across, as when totally full in flight it will overflow the R tank(s). I usually switch to R shortly after takeoff and burn 1/2 or so of the fuel out of there first.

If I land with the L tank empty (or nearly so) then leave the plane overnight, the tanks will equilibrate to both have the same level.

There is a single vent in the middle of the cabin roof; it works. Both tanks will feed the engine when individually selected.

Any thoughts on tricks to equalize the fuel flow (I realize this is a common problem with high wing cessnas...)

T.

[Bruce Fenstermacher]

Dick you are correct for the old style valve found in A models and a few of the early B models but not the newer style valve which is pitured.

Looking at the pictureof the newer style valve the spring (20) pushes the ball ( 18 ) against the o-ring (16) sealing the line shut. To open the line the cam (9) pushes ball (15) against ball (20) which is moved against the spring and away from the o-ring opening the line.

If there was no cam or the selector is in the off position the springs will hold the balls agains the o-rings and close both sides. Turning the selector moves the cam into a position of either opening one side or the other or both open or both off.

So assuming the o-rings are good on both sides the fuel will be completely shut off and not transfer between tanks when the selector is in the off position. Fuel will be one to the engine from either tank selected but not transfer between tanks when either on the right or left tank position.

[gwillford]

I currently have my newer style fuel selector out for repair due to internal leaks. Same as here, fuel transfer as well as the fuel flow can not be stopped wihen the selector is turned to both off. Upon dissasebly, my valve does not appear to have a retainer, number 17 on the exploaded view drawing as shown on the previous posting. Mine appears to only have a groove that the o-ring fits into. Correct fuel valve? Newer version? Uses the same o-rings?

Thanks for the help

George

[Metal Master]

The retainer is a close tolerance fit. The last one I ordered parts for I purchased new retainers and did not need them. The 0-rings fit into the retainer and support it so the only the outer surface is in contact with the Check ball. The retainer also keeps the O-ring from extruding into the fuel flow path. The retainer is the groove that the O-ring goes into. I have had all of the versions of this "late" style fuel selector apart even the later 182 Larger format the are basically similar. In my experiance.