Venting fuel question

[jrm7997]

I have been noticing that the blacktop under my left fuel vent appears to be being eaten up by fuel. Over the years I have tried to find/see fuel coming out the vent but have not.

The other day my hanger mate called me to say he had just seen about three or four ounces of fuel come flowing out. I'm not the builder so i'm not sure of the piping or routing he used. I'm curious if anyone else has seen this and your opinions on what might be causing this.

Thanks in advance
Jim

2002 RV6-A

 

[RV6_flyer]

There is very little space in the RV fuel tanks for heated fuel to expand without it going out the vent. The only way to fix this is to NOT fill the tanks with as much fuel so that when the temperature goes up, the increased fuel volume does not go out the vents.

 

[jrm7997]

There is very little space in the RV fuel tanks for heated fuel to expand without it going out the vent. The only way to fix this is to NOT fill the tanks with as much fuel so that when the temperature goes up, the increased fuel volume does not go out the vents.

That was my first question to my hanger mate. I'm not flying for a bit while i get my medical sorted out so he's been keeping her in shape. I asked if he had just filled up but he hadn't and the tanks were about half full.

Thanks for the reply and I bet you 99% of the time your answer would be right

 

[Ralph Inkster]

A few possible scenarios:
1- blocked or near blocked vent line. Possible insect or other contaminants are restricting the vent line, or possible crimped tubing. Way to test- blow in the tank thru the filler hole & you should be able to hear a flow of air out the vent line under the fuselage. If there is some loose stuff clogging the line, it may be blown out this way. !!do not apply compressed air pressure to the tank!!
2- internal vent port dipping into the fuel when tank too full (as poster #2 described). Don't fill tank quite as full when parked.
3- possible leak into vent line. Not common but been known to happen. There might be a flared fitting inside the tank that might not have been tightened during construction, would cause fuel to leak out of the tank thru the vent tube. How to test- temporarily plug the vent inlet (1/4" tube should be inside & just behind the filler inlet at top of the tank), apply low air pressure to the vent outlet under the fuselage, If you hear air bubbling inside the tank- a loose fitting is the culprit.
These are the ones I can think of right not, maybe others have other suggestions...

 

[jrm7997]

A few possible scenarios:
1- blocked or near blocked vent line. Possible insect or other contaminants are restricting the vent line, or possible crimped tubing. Way to test- blow in the tank thru the filler hole & you should be able to hear a flow of air out the vent line under the fuselage. If there is some loose stuff clogging the line, it may be blown out this way. !!do not apply compressed air pressure to the tank!!
2- internal vent port dipping into the fuel when tank too full (as poster #2 described). Don't fill tank quite as full when parked.
3- possible leak into vent line. Not common but been known to happen. There might be a flared fitting inside the tank that might not have been tightened during construction, would cause fuel to leak out of the tank thru the vent tube. How to test- temporarily plug the vent inlet (1/4" tube should be inside & just behind the filler inlet at top of the tank), apply low air pressure to the vent outlet under the fuselage, If you hear air bubbling inside the tank- a loose fitting is the culprit.
These are the ones I can think of right not, maybe others have other suggestions...

Awesome. During my annual I always give them a little blow and they seem to be clear and the same from side to side. But i think with this, Ill do it again with a little more concern to the sounds.

These are good thank you.

 

[PilotjohnS]

I fill my tanks till it just touches the bottom of the fuel cap. Even then I sometimes see fuel spit out.
I wonder if this could be the reason my Red Cube calibration seems off???

 

[D-Dubya]

Was your hangar mate positive it was coming out one of the vent lines, or could it have been coming out the sniffle valve outlet? When I park my plane, I typically leave the mixture about halfway open with the fuel valve shut (so it won't over-pressure the fuel pressure sensor). Heat soaking after shutting down sometimes forces a little fuel out the sniffle valve outlet.

 

[Ralph Inkster]

I fill my tanks till it just touches the bottom of the fuel cap. Even then I sometimes see fuel spit out.
I wonder if this could be the reason my Red Cube calibration seems off???

The top end of the vent tube is just in behind the tank filler hole, should be against the tank top skin. Do you think your topped off fuel is draining out the vent lines? Fuel will expand a fair bit of it warms up.

Up here where we are, if we top off our tanks in cool afternoon or evening than park the plane in a nice warm hangar, it's likely we'd come back the next morning to find what appears to be a few cups of fuel on the floor. We learn pretty quickly to not fill the tanks right to the top, leave some expansion space in them.
If we need the full capacity of the tank for our next planned trip, best to top off just before take off.

 

[pa38112]

The "Standard" design of the vent lets fuel get trapped and then pushed out by expansion in the tank. It is normal. It can be fixed a number or ways. Search some of the threads.

 

[PilotjohnS]

The "Standard" design of the vent lets fuel get trapped and then pushed out by expansion in the tank. It is normal. It can be fixed a number or ways. Search some of the threads.

The best way is to fly the plane so the fuel in the vent line gets sucked back in. Just another excuse to fly. And the faster you fly, the more effective.

 

[Saville]

I had some vent dripping and the problem was that the B-nut on the vent connecting it to the rib had backed off entirely. I figured it out by rigging a tube on the exterior vent, putting my ear on the wing over the vent, and blew through the tube. I could hear bubbles.

So, after draining the tank, I removed the plate that the sender was on and stuck a camera in to have a look. Sure enough the B-nut was totally backed off. See attached picture. I created a bent wrench and tightened the B-nut. Leak stopped.