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How to test fuel vent

BloomerJohn

BloomerJohn

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My 9A has the standard fuel vents from the inboard end of the tanks into the cockpit, up, forward to the firewall, then down and out to a AN nipple that has been ground to an angle. Question: Should I be able to blow air from the filler and/or the vent nipple through the vent system if the tank is empty?

I am looking for a fuel leak that seems to have recently migrated into the cabin and fouled the floor mats on the right side. A clean dry white paper towel can not find any trace of fuel along the vent line. The leak area is nowhere near the fuel valve..it is along the right side.

Fuel smell is evident with full tanks. None when less than 1/2 tanks.

I have (and will do again this week) checked the tightness of all the vent lines inside the cabin.

Any ideas will be appreciated.

Cheers,

John Nielsen
 
The vent line is rather long and not all that large in diameter so blowing on it will definitely have some resistance.
Because of this, it can be difficult to tell whether it is flowing air.
An easy way to tell is to remove the fuel cap and when blowing into the fuel vent, look for fuel vapors to boil out of the open filler hole.
This test can be done using an air nozzle as long as you hold the air nozzle 3 inches or so away from the fuel vent inlet.
My guess is that the source of your leak is something other than the fuel vent line.
 
Last edited:
OP said the floor matt on the right is fouled. Fuel could find a way in running onto the bottom skin but it would have to get past the spar to soil the floor mat. Totally possible, but I would expect to see a stain on the belly somewhere.
Pull the wing root fairing and look just in case.
 
Leaking Bnut on main fuel line at the tank can cause this exact symptom. Fuel follows the main line into the cabin and will drip where the line drops down right on the carpet. May not even see it under the wing root but a quick wipe around the Bnut with a white cloth and it will show.
 
Sometimes the path out isn't where the source is coming from. I had a leak from the fuel selector valve show up on the right wing root. It followed the spar to the slightly lower wing and out the wing root. Keep digging until you find it and get it corrected.
 
Well I found the problem. Prior owner somehow got a drill too close. Easy fix. just make a new fuel vent line for inside the cabin. I did check to see if I could pass air from the outside vent openings and was able to do so and hear the air come into the tank with the caps off. So now I know they breathe free. To locate this leak, I filled the tank to the top with cold fuel, pushed the plane into the direct hot sun, and inside of 15 minutes fuel was dripping out of the vent line into the cabin were the staining was. The hole is about 6-8" above the level of the top of the tank, so it would only occur with full fuel AND required the expansion of the fuel enough to push it into and up the vent line where this hole was. This will also solve the slight fuel smell I have experienced with full tank on the right. Thank you for the help.
John
 

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