What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

fuel air vent / siphon

seagull

Well Known Member
Can someone explain how the fuel vent airline is supposed to prevent siphoning of fuel.

I filled the airplane for a long flight yesterday. The fuel level was mid way in the neck. During the day the fuel expanded and started the overflow, apparently it became a siphon with the overflow line. This morning it took 2 gallons to top off the tank.

I thought the air vent line was supposed to eliminate this.
 
There is approximately a 1% change in volume of gasoline for every 10.5°C change in temperature (19°F).
Supposing that the temperature of the gasoline raised 40 degrees F. 20 gallons would increase in volume to 20.4 gallons.
 
There is approximately a 1% change in volume of gasoline for every 10.5°C change in temperature (19°F).
Supposing that the temperature of the gasoline raised 40 degrees F. 20 gallons would increase in volume to 20.4 gallons.

I realize the volume increase will start the overflow draining the excess gasoline.
It should stop when the excess is removed, instead once it starts flowing it will remove all the gas above the fitting in the tank because it becomes a siphon. As I understand it the air vent should break the siphon so only the excess (volume increase) is removed.
 
Check the fitting

I had a similar experience.
Check that the anti-siphon line is not blocked, kinked or crimped.
Also. peek behind the bulkhead to be sure that the T-fitting is still upright. Mine had rotated to the side and downward which allowed fuel to enter when overfilled and fuel warmed up.
Good luck!
 
And one more thing...

I determined where the high point of the vent should be so that the flexible tube is relaxed and unlikely to kink, then I fabricated a small clip to support the tube so it couldn’t flop over again. One rivet thru the clip and fuselage skin did the trick.
 
Thanks Dave and Scott,

You nailed it. The air vent line had become disconnected from the "T" and the "T" was turned down. There was evidence fuel had been on the inside skin coming from the "T" . I believe that has been my gas smell when the canopy is slightly open, I'll know for sure after a few flights.

I added 6" of line and raised the loop well above the "T" intersection and zip tied it in place. I tested by overfilling the tank into the neck, the overflow drained out the bottom and stopped when the level was 2" below the cap.

50133241723_d330f8e259_c.jpg
 
Last edited:
my siphon issue

My RV-7 QB would siphon when a full tank was then left in the sun and volume expanded... BUT...it kept up even after the fuel level was below the filler neck where the vent line terminates. It seems that the QB tanks did not have the "B" nut in that tank on the vent line more than a few threads, not even finger tight. and yes, I should have checked it! Would have emptied the tank if I had not noticed the drippings! Turns out, I'm not the only one with QB tanks that had improper vent installations. A few hours later, continued to have fuel leakage. Problem #2 was a seep in the pro seal of the QB tank. Vans fixed it, all better!

Gary Brown
 
Back
Top