1981 C-172 Tanks don't drain evenly

Re: 1981 C-172 Tanks don't drain evenly

Note that the Cessna letter does NOT apply to C-172's !

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Re: 1981 C-172 Tanks don't drain evenly

I Agree - SE81-8 does not specify the C172 but I would certainly check the C172s Service Manual to see if there are any uneven draining procedures listed.  If it ever was a problem with the C172, I am sure Cessna would have (or should have) addressed it as they did with SE81-8.  Jack

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Re: 1981 C-172 Tanks don't drain evenly

Jack, the owner's manual doesn't address my problem directly, but makes reference to "selecting left or right while fueling", etc. so that fuel doesn't get off balance.  I think they figured it out years later--the "unvented fuel cap" shown on the left tank in my handbook would be impossible to replace today, since all they handle now is a "vented" model....which I just bought and will replace this weekend, hopefully to solve my problem.  Sure would be nice if the Cessna folks would publish something after the fact to address things like this...guess that's a problem with litigation, etc.
Bob

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Re: 1981 C-172 Tanks don't drain evenly

Bob, I am curious.. What type of Venting do your tanks have.. Was it just in the fuel caps?   Jack

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Re: 1981 C-172 Tanks don't drain evenly

Just the standard 172 vents--one vented fuel cap on the right tank and an under-wing expansion vent under the left wing behind the strut.  That is how the plane was made in 1981 and is shown in the POH.

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Re: 1981 C-172 Tanks don't drain evenly

I need to dig through some old Cessna Service Instructions.. Seems like I recall one addressing the cap vents.  Just a hunch - but I will research them and see if it is revelent!

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Re: 1981 C-172 Tanks don't drain evenly

I have noticed that almost everyone else has a problem with the fuel draining from the right wing before the left starts emptying.  I recently bought a 1978 172N with a 172P engine.  The problem I am having is that the left wing will empty to 1/4 before the right wing begins emptyingl.  On my last flight, I put the selector on right about 1 hour into the flight, then the left emptied out at about the same rate as the right.  It almost seems to me as if the selector valve is one position off (i.e.; when the selector is pointed to both, it is actually in left and when the selector is positioned to right, it is actually in both).  Is this a possibility?  Anyones thougts on this would be greatly appreciated.
Rich

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Re: 1981 C-172 Tanks don't drain evenly

Rich,
I would doubt very seriously that the selector is at fault.  My 1972 182P exhibits the characteristics that you describe for your plane (my left tank empties first).  The external vent tube is on the left wing (underneath and somewhat behind the strut).  With the selector on both or right only, the left tank will begin to drain.  I assume that with the selector on the left, the left tank will begin to drain also, although I have never tried it.  With the vent tube on the left tank and a connection tube between the two tanks, the pressure differential will push the fuel (or possibly get a siphoning effect) from the left to the right.  Fuel will travel from left to right until they reach a level that is lower than the connector tube and then, you can select the one you want.  Until it reaches that level,regardless of left, right or both, the fuel will move across to the right.

I take off on both, soon switch to the right and switch back to both for landing.

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Re: 1981 C-172 Tanks don't drain evenly

To finish this thread up, I installed the new vented fuel cap on the left tank today, filled both tanks, and flew for 1.6 hrs.  Both tanks drained from the start, the left still slightly behind the right but they ARE now draining from the start.  So, now I have 2 vented caps instead of the original config. of 1-vented and 1-non vented.  This seems to be the answer--anyone else having the uneven drain problem, I recommend replacing the unvented with a vented.
Bob

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Re: 1981 C-172 Tanks don't drain evenly

Rich, in order for the selector to be so misaligned, it would likely require the handle to be mis-installed.  This is ordinarily impossible because the square shaft only has one hole drilled in it to accept the cotter pin/safety wire. 
But ...just in case someone has "monkeyed' with your valve, I suggest the following.
Start your engine and run it ON THE GROUND for two minutes then switch to OFF.  THe engine should die in about a minute or so, thereby confirming the handle is correctly positioned.  Any other result should prompt you to consult a qualified A&P mechanic to correct the problem.
Good Luck.

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Re: 1981 C-172 Tanks don't drain evenly

I still have the original blind cap on the left and vented on the right. They work well as they should. The left tank has the vent tube coming out just behind the strut. That vent tube should let the air in freely and out with some restriction, if not that's why you don't have drainage. It's not bad to have vented cap on the left tank too, but that is just masking the problem with the vent tube. That tube has another important function and if it is not clean then there is another type of problem waiting to come up.

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Re: 1981 C-172 Tanks don't drain evenly

I just bought a 1976 Cessna 150M whose left tank drains much more than the right.
Yesterday: Topped off tanks.  Selector on "Both" (as the only choice is both or off).  Both fuel caps are vented type.   Flew 1.4 hrs.  Became worried that right tank wasn't providing any fuel (based on gauges) -- landed early (after ~1.4 hours) to check fuel.  I topped off both tanks.  7 gallons added to left,  4 gallons added to the right.  Repeated with another 1.4 hour flight.  Approximately the same.

Questions:
1) Anyone confident that once the left reaches 0 (or close), the engine will continue to feed from the right?

2) Anyone know why I'm using 7.8 gph?  It's a new engine with less than 8 hours on it.  Flight was high throttle, leaned, but never much more than 2600 RPM (which according to the POH, should not have taken much more than 6.0).  Should a new O-100 be drinking 100LL so fast?

3) If something IS inhibiting flow from the right tank, what could it be if not a vacuum/venting issue?  Is there a fuel filter/screen that could be clogged?  Bad fuel line?

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Re: 1981 C-172 Tanks don't drain evenly

Quote: "Should a new O-100 be drinking 100LL so fast?"

Oops...  I meant O-200 (not O-100).  The engine is already small enough.  I don't need to belittle it even more. wink

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