1981 C-172 Tanks don't drain evenly

1981 C-172 Tanks don't drain evenly

I have noticed recently that after flying for an hour, I have to add 8 gallons to the right tank and 0 gals. to the left.  This has happened the last 3 times; I know these tanks are famous for uneven draining but it almost looks like the left tank ISN'T draining at all.  Anyone have any thoughts on this, and how to determine if fuel is draining?  Thanks in advance.

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

I experience the SAME thing in my '56 skyhawk.  Seems pretty common.

     Dave

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

DO YOU FLY LEVEL WITH BALL IN THE CENTER ?

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

Does anyone know if putting vented caps on both tanks will solve this, and is it legal?

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

<HTML>I had problems with my 150 and went to both caps being vented--took care of the problem. I don't think you can buy a non-vented cap anymore and that should tell us something. I had the same problem early on with my 172 and corrected it the same way. There are several other areas to check though--the vent behind the left strut and the valve in the left wing can also play a serious role</HTML>

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

To check if your tank is draining.  One simple approach would be to select the tank of interest from the inside and pull the fuel strainer open and drain fuel into a clean bucket.  If you can drain a few quarts of fuel I would say it would be OK.  (If you like science experiments).... You can do the other tank and compare flow rates if you suspect a restriction of somekind.  I would do this also with the vent caps on the tanks loose so venting problems can be eliminated in this part of the experiment.  Then do it again with the caps on tight.  If you notice a significant difference venting could be problem.

The assumption here is that the fuel tank selector valve is good.  To gain some confidence in it, try selecting the fuel selector valve to off and pull the strainer til fuel stops flowing.  Then select the tank of interest.  Time and measure the quantity of fuel and then do the other other tank. 

One last caution about selecting off.  After owning my plane for a short time, did just that.  Then I noticed an unisolable leak from the valve.  What a mess.  of course it was a sunday. So I rigged a 5 gallon drip pan and overflow into a 10 gallon gas can.  Had to wait til the maintenance came in on Monday to deal with the mess.

The selector valves are not frequently turned to the Off position and there o-ring seal was aged so it lost it's resilience.  This resulted in the inability to seal once the selector switch was moved to a new spot on the old o-ring.  So do this test when you are prepared to deal with a leak.

my two cents

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

Barry, you are not alone. I did the same thing, with the same results. My valve leaked in the off position. Fortunate for me that when put back in the both position, the leak stopped.

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

Bob,

What was the outcome?

JimJ

When I tried turning it back to both it started to leak faster.  I found the selector valve position that gave me the minimum leak - drops per minute.  For a while I kept my empty, plastic collection rig tied to my tie downs, just in case.  Someone ran over it when I was gone and that ended that.  Fortunately, after a new o-ring was installed the problem was eliminated.

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

Thanks everyone for the input.  I will try the test this weekend of draining each tank with the fuel strainer while selecting it with the valve.  If it drains normally, that would indicate no obstructions, but I will most likely install a vented cap for the left tank also.  What could it hurt?
Bob

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

What approval data are people useing that decide to modify their certified fuel systems?  I would be very cautious about putting a vented cap in the low pressure area of the wing. (which is where most caps are)

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

<HTML>
Cessna does not carry the non-vented caps any longer---I think they are telling us they made a mistake in the first instance by providing one vented and one non-vented,  In any case I know of no one still using only one vented cap, and no one I know except Bob is having fuel flow problems--including me--that has to tell us something however my circle of Cessna owners is small.</HTML>

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

Thanks Ernie--where's a good place to order a vented fuel cap?

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

<HTML> Bob --- my maintenance shop recomended the vented caps for both tanks and got them for me. I recall the cost about 35 bucks--that was two years ago for my 172 and over three years ago for the 150.  The shop is still recomending two vented caps. The shop also advised me that you can't get a non-vented cap from Cessna--so my best guess is they ordered the caps from Cessna.. The question regarding the authority for two vented caps----I can't respond to the legality of it---just that it corrects a problem encountered by a lot of cessna owners. Another major problem area can be improper placement of the vent behind the left strut.</HTML>

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

HIgh-wing Cessna's are notorious for one wing seeming to drain somewhat faster than the other.  Don't let this bother you. 
  In most cases it is only due to tank venting anomalies and/or siphoning from one side to the other. 
  Example: The right cap is vented the left is not (or the left underwing fuel vent services both tanks via a cross-over vent line.)  This results is slightly different vent pressures, but more likely the truth is that both tanks feed the engine equally, but the cross-over line also siphons fuel from the right tank over to the left tank for various reasons.*  The result is the right tank READS lower than the left because the left tank was somewhat replenished by the right tank via the crossover line.  After the right tank fuel level drops below the ability to supply the crossover vent line with fuel, both tanks will continue to feed the engine equally and both fuel levels will drop at similar rates.

*-The various reasons are: The right tank cap is vented while the left is not, so a slight suction can exist to siphon fuel from right to left.  Or the right rudder applied during climb (also happens to coincide with the highest fuel consumption period early in the flight) causes the right tank to keep it's fuel pushed up against the crossover line thereby allowing a siphon action to resupply the left tank with fuel from the right tank, etc. etc.
  In any case, notice that after a while both tanks feed the engine just fine.  Keep notice on your time in flight vs the fuel consumption rate of your aircraft, and you'll see that both tanks actually feed the engine just fine, and that this is not a real problem.

  Dual vented caps may or may not solve the problem but there is no danger of fuel sucking out of the cap vents because the caps are in a low pressure area.  The vent valves are one-way valves.  They let air in, but don't let fuel out. 
  (However, a cap's gasket can become dry and cracked, and can leak fuel out past the cap due to the low-pressure area.  So inspect and keep your fuel cap gaskets in good condition.)

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

Oh yes, and about the fuel selector valves:
  Most pilots never operate their valves to the off position.  I feel this is a mistake.  It's an important safety item to ensure that your fuel valves work properly, especially in the OFF postion.  An off-airport landing always needs to have the fuel valve selected to OFF.  Parking the airplane for overnights should also have the fuel valve selected off.
  One reason valves fail (such as has been mentioned here) is lack of use.  The seals harden and debris bulds up and then someone turns it to a position in which it's not accustomed and the debris injures the dry seal and ....viola!   a Leak!
  I put my airplane into the hangar and turn the fuel selector valve off EVERY TIME!  I'd never trust a carb float valve needle to keep all 42 gallons of fuel in my tanks instead of allowing it to drain all over the hangar floor where it can blow up the entire hangar and it's contents.  Fuel valves have an off position for a reason.  By using it regularly, I'm assured it will work when I need it.

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

George,
Thanks for your wisdom, esp. re. using the "off" position.  However, my concern re. uneven tank draining is that if it takes 1-2 hours of flying before the tanks decide to drain evenly, I am, at that point, about 70-80 lbs. heavier on one side than on the other, creating an "out-of rig" flying scenario.  And, if I fly a long distance without adding fuel, I will be flying on only one tank towards the end (i.e. Cessna POH warnings  re. landings on one tank).  I think I will install a vented cap on the other tank to see if this helps the draining.
Thanks again for your response!
Bob

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

George
  you said 'VIOLA'
         did you mean 'VOILA'   ?

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

Daniel...
I think that WAS supposed to be Viola...the musical instrument...
like in the scene of a movie when one opens a door and has a
startling surprise..the background music just happens.  In this case it was the sound of a viola...it's like a big fat violin...

I glad to say I haven't seen "walla" in print for awhile.. ;p

Michael

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

Voila!!  Michael got it!!  (grin)
  What did you expect, Daniel, from a state that educated our dyslexic leader? (gren) ;Þ

Bob, a simple solution is to take off on Both, then switch to the slow tank (left?) and let it drain down a bit until it indicates less than the other, ...or operate on it for one hour, then switch to the opposite for one hour, ....then operate on Both for the rest of the flight, etc.

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

George,
That would work but I don't trust ANY fuel gauges.  I ordered a vented fuel cap yesterday from Hill Aircraft in ATL (Cessnaparts.com) and received it today!  I will let everyone know if this solves the problem.  Thanks to all.
Bob

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

<HTML>Bob--I don't think George was suggesting you trust the gauges--quite the opposite--start with full tanks on both after -- after you are confortable in flight go to a single tank for one hour (timed) switch to the other tank for one hour (timed) and then back to both---no trust involved. Hope the second vented cap along with proper positioning of the vent behind the left strut solves your problem</HTML>

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

Cessna created a Service Information Letter for 1979 and Later 180, 182s.
Subject: Fuel System Venting Adjustment.  (Uneven Fuel flow)

Letter number - SE81-8

This letter  gives instructions to adjust vents if the fuel tanks levels are greater than 5 gallons apart.

If you can find a copy of this SE - it might be your answer..

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

Is someone locates the Service Info Letter from Cessna, a way of sharing that to this thread, I'm sure would be appreciated my others and myself...Thanks :-)
Might that be posted on a website somewhere?  FAA..Cessna??

Bob wrote...
Cessna created a Service Information Letter for 1979 and Later 180, 182s.
Subject: Fuel System Venting Adjustment. (Uneven Fuel flow)

Letter number - SE81-8

This letter gives instructions to adjust vents if the fuel tanks levels are greater than 5 gallons apart.

If you can find a copy of this SE - it might be your answer..

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

I will enter it (word for word) on this site later today for everyone, if it can't be found on the web.  It was Cessna's method of   adjusting of the vents to solve uneven draining.  Jack

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

See Reference posted 03-04-04 for Cessna Service Information on Fuel System Venting Adjustment SE81-8  jack

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