2 1/2 gals leaked out the fuel vent line

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422Phil

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422Phil
Flew my 1977 Cessna 172 from FL to Tenn recently. Re-fueled the tanks, parked it at the FBO (sits at a steep grade). Returned 4 hrs later to find fuel draining at pretty decent rate out of the fuel vent. Has anyone replaced their fuel vent line with the vent that has a hump to place the line above the tank? If so, did it work, and how much mechanic time was involved to compete the swap?
 
Depending on year, your POH probably has a line or two in there about parking across a slope and turning the fuel either to off, or to one tank only.

From a quick google search of that years POH, I found "To ensure maximum fuel capacity when refueling, place the fuel selector valve in either LEFT or RIGHT position to prevent cross-feeding."
 
Yep. My club policy is to leave the selector in left or right when parked. Otherwise, given the slope, fuel runs out the downhill vent.
 
I was always taught to leave the selector on the left tank when leaving the plane or fueling

One more reason I hate the 172.. you have two separate (but not really!) tanks









/sorry, I couldn't resist
 
Thanks for the tip. Hoping to hear from someone that has replaced the vent tube with the McFarlane solution or other product.
 
422Phil said:
Thanks for the tip. Hoping to hear from someone that has replaced the vent tube with the McFarlane solution or other product.
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That humped tube will help some, but on a serious enough slope it will still drain fuel.

There's another factor. The vent check valve inside the tank is supposed to be positioned so that it is angled forward and upward near the top of the tank. The hole in the tank wall for that fitting has a flat on one side to index it, but the metal is thin and soft and if a mechanic isn't careful when tightening the tube fittings he can overcome the indexing and rotate the valve so it sits well down into the fuel, where it will siphon off lots of fuel.

upload_2022-2-24_20-48-58.png
 
Hump worked well on a 152. Some folks carry a small ramp made from 2 x 6 s.
Select LEFT or RIGHT tank at Selector.
 
422Phil said:
Flew my 1977 Cessna 172 from FL to Tenn recently. Re-fueled the tanks, parked it at the FBO (sits at a steep grade). Returned 4 hrs later to find fuel draining at pretty decent rate out of the fuel vent. Has anyone replaced their fuel vent line with the vent that has a hump to place the line above the tank? If so, did it work, and how much mechanic time was involved to compete the swap?
Click to expand...

What position was your fuel selector in when you parked it?

The anti drip vent line from McFarlane is not going to prevent this from happening if parked out of level with the fuel selector on both.

In most cases removal of the tank cover on top of the wings in necessary to install the McFarlane vent line. That can be a major challenge getting the screws out of the cover. It is also a major job at least is on my 1980 N model. They say you might be able to install it through the inspection cover, no way on mine.

Also there is no check valves inside the tanks of my N model, just strainers. No check valves. I had mine out and apart.
 
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422Phil said:
Both.
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My suggestion would be always put the selector on left or right or off is you have it when parking.
It keeps both tanks from draining if one sprang a leak. And keeps it from draining side to side.
 
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I think the poh says to do what everyone is suggesting. Leave it in left or right. I’ve had a 172M for well over 20 years and almost never do this, but I normally park in a level spot. Occasionally when it makes sense I will, otherwise it stays in both. Never had an issue other than a few drops here and there.
 
Jdm said:
I think the poh says to do what everyone is suggesting. Leave it in left or right. I’ve had a 172M for well over 20 years and almost never do this, but I normally park in a level spot. Occasionally when it makes sense I will, otherwise it stays in both. Never had an issue other than a few drops here and there.
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The previous owner of my plane told me he always left the fuel selector in both. He did not fly the plane much. When I got it the fuel selector was stiff and didn't move very easy. 1000 hrs later and plenty of lube it has loosened up but needs to be resealed because when I shut it off to clean the strainer if still drips a little.
 
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Gary Ward said:
In most cases removal of the tank cover on top of the wings in necessary to install the McFarlane vent line. That can be a major challenge getting the screws out of the cover. It is also a major job at least is on my 1980 N model. They say you might be able to install it through the inspection cover, no way on mine.

Also there is no check valves inside the tanks of my N model, just strainers. No check valves. I had mine out and apart.
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Removal of that cover is supposed to happen every three years or 1000 hours, according to Cessna's recommendations. They mostly never get removed until the tank springs a leak, and that leak is often due to the rotted-away rubber bumpers under the tank and on the hold-down straps, and the tank gets chafed through. Getting the cover screws out, if they're corroded in there badly enough, can cause damage to the wing main spar. Owners saving money no matter how much it costs in the end, and that's often due to the mechanic failing to educate the owner.

This is from the service manual for your airplane model and year:

upload_2022-2-25_11-7-29.png

There is no indication that your airplane should be missing the vent check valve. It's in the left tank only and connects to the underwing vent line.

http://www.aeroelectric.com/Referen...anuals/Cessna_172_1977-1986_MM_D2065-3-13.pdf

Chapter 12 of the manual.
 
Jdm said:
I think the poh says to do what everyone is suggesting. Leave it in left or right. I’ve had a 172M for well over 20 years and almost never do this, but I normally park in a level spot. Occasionally when it makes sense I will, otherwise it stays in both. Never had an issue other than a few drops here and there.
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Up until the later N models, you could leave it in the OFF position and it would prevent crossflow. The valve in the older 172s was a cam-operated affair, not a plug-typ valve that will crossflow in the OFF position. When it's off it's really off.
 
Gary Ward said:
The previous owner of my plane told me he always left the fuel selector in both. He did not fly the plane much. When I got it the fuel selector was stiff and didn't move very easy. 1000 hrs later and plenty of lube it has loosened up but needs to be resealed because when I shut it off to clean the strainer if still drips a little.
Click to expand...
In 172s older than yours (up to M models) that valve has a series of O-rings in it. Some of them act as valve seats, and the valves themselves are a series of plated steel balls commonly used in ball bearings. It's an easy rebuild; the biggest hassle is that the tanks have to be totally drained. There are no shutoffs at the tanks themselves that would make such maintenance much easier. Your valve looks like this:

upload_2022-2-25_11-16-53.png

Bits of debris have likely scored the rotor and/or seals. That plug in the bottom, #13, should be periodically removed to drain junk out of the valve. There should be a hole in the belly skin underneath it.
 
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