Fuel valve

[leighton]

A little confused when reading posts on the fuel valve - the question is - on a 1953B will fuel level equalize between tanks when on level ground when in off position. I thought I had a problem as valve was not shutting of completely but after cycling a few times it is functioning properly (shutting off) but fuel is still equalizing between tanks. I did put more fuel in one tank as test, tanks are less the 1/4 full. Hoping I don't have to replace the valve !!

[n2582d]

I just double-checked my rebuilt fuel valve. In the off position all three ports are blocked. There is no flow between the left and right port. There is no need to replace the valve; it is easily rebuilt. Getting it out and reinstalled is another story. If you have it rebuilt be sure and read up on how to do it as the IPC shows some parts in the wrong sequence.

[GAHorn]

The valve has a "cam" beneath/attached to the selector. When in "Both" position, each tank will supply fuel to the gascolator-side of the valve, as that cam opens a ball-type check-valve. There is nothing to prevent fuel from backflowing up into a tank except the weight of existing fuel within the tank. Long term storage of the airplane with the valve in the "Both" position may allow some fuel to transfer S-L-O-W-L-Y between tanks ("equalize" as you wrote). But it is unnoticeable in most cases because, again, of the weight of existing fuel.

The Cessna fuel system design has undergone several changes over the years. The "gooseneck" prevents one tank from overflowing the other tank due to "equalizing" because the gooseneck is higher on the airframe. When Cessna changed to a lower fuel vent beneath the left wing, they had to install a check-valve to prevent fuel from overflowing onto the ramp. (Failed check-valves allowed the problem anyway,...along with other occasional problems regarding vent-pressure and anti-icing placement behind the left wing-strut, ...and rubber bladders contributed even more problems with that issue even when the valve was turned "Off".... as the bladder collapsed due to failed venting it might actually reverse-flow fuel out the vent....but that's another discussion, not applicable to 170's.)

If you are leaving your selector on "Both" while in storage, a failed valve...or more likely...a stuck carb float-valve....can let your tanks empty onto the hangar-floor. (I hope your insurance company will protect you adequately from any claims by other hangar occupants and owner if/when the fire breaks out. And that no one is hurt.)

Some folks are reluctant to close the valve to the "off" position because they fear "unnecessary" valve wear. The machined-body valve used on SN's above 20285 (and perhaps retro-fitted to others) is not harmed by use, in my opinion. In fact, the valve should be turned "Off" during storage to prevent mishaps, as well as to confirm the valve operates properly to shut-off fuel in case that is ever needed due to an engine fire or failure of the fuel-system downstream of the valve. (I have witnessed several events where the pilot/owner valiantly and frantically attempted to stop a fuel-overflow onto the ramp/ground due to such failure. Rapid access to the failure area (the carburetor) is not easy with the cowl in-place. If this happens to you... you will have a serious hazard of fire and can lose the airplane if not life itself. The pouring-fuel rapidly runs down the pilot's arms and soaks his clothing, not to mention the rapidly-enlarging puddle of gasoline in which he's kneeling.)

Closing the valve during storage also re-spreads lubricant and excersizes the components, which I feel are good for them. "Use it or Lose it", IMO.

I posted a copy of the article I authored in this thread which (article begins down-the-page) and includes pics:
http://cessna170.org/forums/viewtopic.p ... e&start=45

A more direct link is: http://cessna170.org/forums/viewtopic.p ... hem#p72283

(The article also appeared in The 170 News, 1st Qtr 2011) for those who keep them.

[Bruce Fenstermacher]

For those of us who are lucky enough to own an A model and even a B model 20285 and prior with the brass cast Weather Head valve, the value sequence works completely different. On both fuel can flow from either tank to the gascolator and fuel will equalize between tanks. In either the left or right position fuel will flow from which ever side is selected only to the gascolator. Fuel can not equalize between tanks. In the off position fuel can flow (equalize) between tanks but DOES NOT flow to the gascolator. '48s have two completely different fuel valves which I can not attest to the flow through of each of the valves. I believe one only has left or right or off. The other might have a both and an off in which case it probably acts like the A model valve.

[leighton]

Thanks all for the fast response and great artical George, pictures are most helpful.