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Fuel Cap Sucking and Blowing

S

StuBob

Well Known Member
Carbureted O-360-A2A.

My fuel tanks - right more than left - if the caps are left on tight, will often “whoosh” air in or “poof” air out when the caps are removed. The more fuel in the tanks, the worse it is. They occasionally dribble out the drains, but not a lot and not always. The person I bought it from says the owner previous to him had the tanks resealed at some place that advertised that for RV tanks, and that it’s done this sucking and blowing ever since then, but not before.

My suspicion is that there’s ProSeal in the vent line, or maybe the end of the vent line isn’t bent upward enough to keep it out of the fuel when the tanks are full.

I’m at a loss as to how to troubleshoot this, short of opening the tanks and taking a look. Meanwhile, I’m nervous about flying IFR and the like, even though this has apparently been going on for 2 years or more.

Thanks for reading/helping.

Stu
 
You should be able to disconnect the vent lines at the wing root and rig up a temporary tube/fittings to gently blow into the (1) fuel tank vent plumbing, and (2) fuselage vent plumbing. If you feel resistance you'll know where the blockage is (tank vent lines or fuselage vent lines).
 
Remove the fuel cap, attach a rubber hose to vent fitting on belly and blow. There should be no restriction in the line.
 
Jake's method is absolutely the easiest starting point. You might want to have someone listen at the open fuel cap and see if they hear air and feel it coming out when you blow on the hose. If it's restricted, then you've got to trace that 1/4" vent line out to see if where the restriction is. It could be a dirt dauber or a kink in the line between the fuselage and the tank that would be easy to fix. But if it started when the tank was "resealed", chances are it has something to do with that. An issue inside the tank can be fixed as well, but it's a little more complicated.

I'd be reluctant to fly until you determine what's going on. Something ain't right.
 
Fix before fly for sure..

As others are mentioning, the vent line is likely the problem. There are historical threads on this issue, and more than one RV has suffered fuel starvation in flight and visible collapsing of the tanks when the fuel pump draws negative pressure. Mud daubers are a known issue plugging vents, and Proseal (especially if recently re-sealed) are common culprits. I regularly check mine by backblowing with the cap off.
 
StuBob said:
Carbureted O-360-A2A.

My fuel tanks - right more than left - if the caps are left on tight, will often “whoosh” air in or “poof” air out when the caps are removed. The more fuel in the tanks, the worse it is. They occasionally dribble out the drains, but not a lot and not always. The person I bought it from says the owner previous to him had the tanks resealed at some place that advertised that for RV tanks, and that it’s done this sucking and blowing ever since then, but not before.

My suspicion is that there’s ProSeal in the vent line, or maybe the end of the vent line isn’t bent upward enough to keep it out of the fuel when the tanks are full.

I’m at a loss as to how to troubleshoot this, short of opening the tanks and taking a look. Meanwhile, I’m nervous about flying IFR and the like, even though this has apparently been going on for 2 years or more.

Thanks for reading/helping.

Stu
Click to expand...

I found a vent plugged with pro seal during my build. The pics are long gone but I was able to use bent hinge pin and bent awls to clear the tube, working through the fuel cap opening. You can read about it here:
https://vansairforce.net/community/showthread.php?t=16771&highlight=vent+line

Thanks,
Joe
 
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StuBob said:
Update: Fuel vents are completely open on both sides. I’m stumped.
Click to expand...

If you tested the vents from the bottom, there may be a flap of sealant that is acting like a one way valve.....

Test from the filler neck with the fuel valve closed.
 
That's worth checking but if that was it, I wouldn't expect it to generate both negative and positive pressure, right?
 
The end of the vent line isn’t bent upward enough to keep it out of the fuel when the tanks are full. This may be the problem.
If so, It would not be an unsafe condition that will cause an engine failure.
Fill the tanks full and remove fuel caps and blow through the vent line.
Then you will know.

Good luck
 
StuBob said:
Update: Fuel vents are completely open on both sides. I’m stumped.
Click to expand...

This sounds like you may have a loose b-nut at the inner fuel tank bulkhead or some other vent line connection issue.

Do fuel tanks build up pressure and continue to vent fuel overboard when the fuel level is below the vent line bulkhead b-nut?

Check out this post - https://vansairforce.net/community/showpost.php?p=1258359&postcount=4

See the picture below. It is from RV-6 plans, but you get the idea.
 

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StuBob said:
Update: Fuel vents are completely open on both sides. I’m stumped.
Click to expand...

If you get positive or negative pressure build up in the tank, IT IS NOT vented, period. Very possible that the vent line is without blockage, but SOMEWHERE from the inside of the tank to the atmosphere is a blockage. Pull the line off the tank and run a piece of safety wire into the fitting feeling for a blockage. DO NOT FLY. If this gets worse, at some point the pump will not be able to overcome the resistance.
 
Just to bring this thing full circle — not that anyone is interested, but I can’t help myself.

I put a length of rubber hose on the inside end of the vent line and blew. No resistance. A helper heard and felt air coming out of the overflow. Then I blew in the overflow end and got air in the tanks.

And now the problem is gone. There must’ve been some schmutz in there that I blew out. I was careful to blow from inside out first, reducing the risk of blowing stuff into the tank, but I suppose that’s a risk. Sumping has yielded nothing so far.

I also replaced both fuel caps, but I don’t think that probably had anything to do with it.

Thanks for the advice!
 
Well, I'm interested

Still trying to wrap my head around what's happening here. Can't even conceive how you'd get a rubber hose around the inside end of the vent line, unless you have trained gerbils to get up inside the tank through the gas cap.:rolleyes:

I'm glad the problem is resolved, but it seems like there should be a smoking gun somewhere. Can you run a tank close to empty, then pull a sump drain to see if anything comes out? A borescope might be useful, but it would ideally need to be rated for use in hazardous environments.
 
It was like a little aquarium hose. The end of the vent is less than a finger’s length from the filler. It wasn’t very hard, really.
 
StuBob said:
Just to bring this thing full circle — not that anyone is interested, but I can’t help myself.

I put a length of rubber hose on the inside end of the vent line and blew. No resistance. A helper heard and felt air coming out of the overflow. Then I blew in the overflow end and got air in the tanks.

And now the problem is gone. There must’ve been some schmutz in there that I blew out. I was careful to blow from inside out first, reducing the risk of blowing stuff into the tank, but I suppose that’s a risk. Sumping has yielded nothing so far.

I also replaced both fuel caps, but I don’t think that probably had anything to do with it.

Thanks for the advice!
Click to expand...

We have had bugs stuck in vents on certified aircraft that needed to be blown out too
 
Isn’t the end of the vent supposed to be at the outer most end of the tank where there is likely to be an air pocket due to the dihedral?
 
StephenC said:
Isn’t the end of the vent supposed to be at the outer most end of the tank where there is likely to be an air pocket due to the dihedral?
Click to expand...

Indeed - if that's not correct on these tanks, there could be other things that are wrong. I'd keep looking to understand the issue. No fuel to the engine can be the start of a very bad day.
 
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