Possible Fuel Tank Leak

Possible Fuel Tank Leak

I have a 1973 C172M.  I've been watching fresh fuel stains appear down the right side of the rear window.  This occurs after topping off the tanks.  I use to chalk it up to overflowing the tank, but now I begining to get suspicius because it appears only on the righ side.  I'm told that it could be the gasket rotting away or cracks developing in the tanks at the top. This tank was replace several years before I bought he plane.  The left side has the original tank.  The gasket at the fill tank was check during the annual in September, so I'd rule that one out.

I'd appreciate any suggestions & comments. About a year and a half ago, I replaced the carbon steel screws with stainless steel screws on this tank.  The screws were torqued to the recommended value.

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Re: Possible Fuel Tank Leak

Barry

You may want to check those screws if I recall reading somewhere that the stainless screws may be a little tighter
thus causing the torque to not actually tighten the gasket seal as required. Maybe retry torque a little higher value.

Bill

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Re: Possible Fuel Tank Leak

Bill, thanks for the idea.

That generated a number of ideas.

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Re: Possible Fuel Tank Leak

Barry,
Also check the sending unit gasket, and look at the sending unit terminal. It's very common for them to start leaking at this location.
Del

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Re: Possible Fuel Tank Leak

Del,

Thanks, your the second person to recommend that possibility.  I'll check it out this weekend.

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Re: Possible Fuel Tank Leak

Barry, 80% of the leaks I see on Cessna fuel tanks are from what Del mentioned above. 

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Re: Possible Fuel Tank Leak

Del or Mystery Guest,

The sender unit is on top of the tank.  Based on the parts manual its under a small cover on top of the tank next to near the gas cap.  Is that right?

I went looking for the leak.  I checked took off the inspection plates and the sheet metal strips at the root of the wing.  Couldn't find the path of the fuel leak?  It's very puzzling.  How does a leak from the sender get on the fuselage rear window?  Maybe some msyteries are not meant to be solved?

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Re: Possible Fuel Tank Leak

Barry

Inside the wing can you see any trace left by the fuel flow to that part of wing where it will run down fuselage at rear window? I would carefully check that area of wing. There should be some staining.

Bill


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Re: Possible Fuel Tank Leak

Barry,
It's possible you may actually have a problem with the tank itself. However I would check the following items first;
1. Sending unit terminal and gasket. Fuel can leak here and run down in an aft and inboard direction to deposit in the lower wing skin just forward of the aft spar. Then streak down the window in flight.
2. The rubber interconnect hoses between wing root and fuselage. These often get overlooked when it comes time to replace engine hoses, but they carry the fuel first on it's way to the engine.
3. The "B" nuts on the interconnect lines near those rubber hoses.

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Re: Possible Fuel Tank Leak

Del,

I appreciate the input, Thanks.

I checked the sender gasket and terminal but no indication of past leakage.

The fuel in the tank does not contiuously leak, it's only after toppping it off.  So I don't think I'll find the rubber hose connected to the fuel line leaking.  If it were lieaking, wouldn't it leak continuously until the fuel level in the tank falls below the leaking hose? Am I missing something?

I believe the leak is near the top of the tank because the tank remains relatively full.  The leak occurs while on the ground, since my cover is stained and I never see wet fuel on the plane, just new stains and smells.

I'm starting to believe it may be the tank.  This tank had been replaced one time before I bought the plane.

My next step.  On one sunny day all the inspection covers removed, I intend to fill the tank up to the very top near the cap.  Then, I'm going to wait and watch for leaks.

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Re: Possible Fuel Tank Leak

Barry

Here would be a far fetched thing....

You indicated that this same tank had been replaced before you purchased the plane. Was there any reason specified for this replacement? Had the tank been rubbing up against a protruding screw or rivet for some reason and it may have made a tiny hole in tank.

Also is the plane sitting level and fuel from left tank is not draining to right tank and maybe going out thru filler cap.

Noted was that you mentiond the cover is stained would indicate that fuel might be going out cap and cover is drawing it to the lower point. Try leaving your cover off and see if it does same. Also try filling left tank and leave right tank maybe a gallon or two lower than normal and see if it doesn't fill from left tank while sitting.

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Re: Possible Fuel Tank Leak

Bill,

Thanks for the feedback - ideas.  The cover I was speaking about is the airplane cover.  It get's stained where the gas runs down the fusealge over the rear passenger window.

Barry

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Re: Possible Fuel Tank Leak

Barry

Since the cover is on top of the wing and fuselage it seems to me that the fuel is on top of wing which cover also may cover fuel caps. If it was just inside the wing, cover may not get stained from fuel. This is why it may be possible that fuel cap is leaking from overflowing.

Bill

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Re: Possible Fuel Tank Leak

I had the same problem. I tryed several things before finding the right solution. Replace the fuel sensor gasket and screws. Use the correct p/n for the gasket and the screws. The screws are special, about $2.oo ea. Dont use any washers if the manual does not call for them. If your sensor has a ground strap, use a fuel proof sealer under the strap around the screw but no where else. Some  dealers sell a screw kit for this repair. If you need more info e- mail me. Rich ia

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Re: Possible Fuel Tank Leak

Rich,
The fuel sending unit has sealed dome nuts, so no fuel will leak past the screws whether they have a wire terminal or washer under the head. The main access also has these sealed dome nuts. However you did cause me to remember another possibility for a leak. If someone installs a screw that is to long it can pierce the dome. Your comment about using the correct screws at all locations is important. More than once I've seen these aftermarket stainless screw kits come with the incorrect length.

Barry,
How goes the fuel leak?

Del

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Re: Possible Fuel Tank Leak

Well the fuel tank is out of the wing and an old repair weld on top of the tank is being repaired again.  Apparently it developed a crack in the same area on top of the tank.  The original crack occurred before I bought the airplane.

The mechanic says that the location of the leak is a common location.  I asked if the area could be reinforce.  He said that it couldn't because of interference with the skin.

I'm not sure exactly where it is leaking, but he said once you take the cover off the leak location was obvious.

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