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fuel exiting from right vent line

jask

Well Known Member
Right tank is full. When I open the hangar door, the sun shines on the right wing causing fuel to exit the vent line after about 10 minutes. Is this likely a problem with the vent line or in the tank?
 
Normal

No problem at all...

AvGas (most fluids...actually) will expand when heated - the expanding fuel travels out the vent and onto the hangar floor.
 
fuel vent

It never happens on the left side when the left tank is full. Why the difference?
 
It never happens on the left side when the left tank is full. Why the difference?

Give it time -- it will :)

This happens on certified aircraft as well (Cessna 210, 182, etc.)

If the hangar floor is slightly tilted, or the aircraft is slightly higher on one side due to tire inflation difference, it may not dribble.

The solution is to not fill up the tank to the tippy top -- leave a 1/4" or so between the bottom of the fuel cap flange and the top of the fuel.
 
venting

Give it time -- it will :)

This happens on certified aircraft as well (Cessna 210, 182, etc.)

If the hangar floor is slightly tilted, or the aircraft is slightly higher on one side due to tire inflation difference, it may not dribble.

The solution is to not fill up the tank to the tippy top -- leave a 1/4" or so between the bottom of the fuel cap flange and the top of the fuel.

The right tank is about an inch from the filler neck.
 
The air in the tank may expand enough to vent a slug of fuel that was already in the vent line from the previous flight.
 
Loose B-nut at the vent line connection inside the tank, inner surface of the root rib. You can easily reach it if you remove the fuel level sender cover plate.
 
The vent line is probably terminated at a less than ideal spot at the end of the run in the outboard end of the tank. I bend mine up and get it terminated with about a 3/32" clearance from the top of the tank skin. What is happening is that as the sun heats up the tank and fuel, the fuel expands a bit and goes above the vent termination point and spills out a bit. just don't top off the tanks 100% and leave a couple gallons shy, then top it off when you want to go fly with full tanks.
 
I agree fully with Dan as I have the same problem. It is squirting a few drops of fuel about every 30 minutes or so, until it reaches a level below the vent line exit point.
 
fuel expands more than i would have thought. i think i calculated about a gallon for every 25. don't remember the temp differences, but it was for a likely real life situation.
 
Easy to diagnose if you can get a fingertip over the end of the vent line, through the filler opening. If so, have a buddy blow air into the vent under the fuselage while you listen at the filler. Hissing or bubbling inside the tank is a vent line problem.
 
Last edited:
Dan has it. I determined that was what was wrong with my right tank. I was able to tightened the nut with the wing on. Not fun but I did it, and it worked.
 
Venting

My aircraft has been in the hangar for about a month while I make modifications to the instrument panel and get the rudder pedals/brakes to work for both of us. I am 6-2 and my wife is 5-2. Anyway, some more info on the venting situation. I initially, the fuel level was about 1 inch below the top of the tank when I filled it. Now it is about 2-3 inches down. That is the amount of fuel that has leaked on the hangar floor in the past month during the heating and cooling each day. I can feel the vent line via the fuel cap hole and the line is attached. Today, I got no leakage of fuel when the sun was shining on the right tank. The lower level of fuel may now be lower than the b nut exiting the tank. It looks like I am going to be making a tool to tighten that nut and hopefully fix the leak.
 
I initially, the fuel level was about 1 inch below the top of the tank when I filled it. Now it is about 2-3 inches down. That is the amount of fuel that has leaked on the hangar floor in the past month during the heating and cooling each day.

Yep, that isn't normal venting...that is a leak.
 
It looks like I am going to be making a tool to tighten that nut and hopefully fix the leak.

Shouldn't need a tool for a -7. Pretty sure you can reach in there through the fuel sender cover plate.
 
venting

Shouldn't need a tool for a -7. Pretty sure you can reach in there through the fuel sender cover plate.

Yes, I looked at the drawing and you are correct. The thread I was looking at was for a 10. Thanks a lot Dan.
 
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