Obstructed tank vent

[Dugaru]

I noticed one of my fuel tanks was gurgling quite a bit and filling slowly. I suspected a blocked vent, but the outlet near the firewall looked okay: I could see that nothing was covering the little mesh screen that had been installed over the end of the vent tube, presumably to keep out insects etc.

But then I looked closer. The screens had been attached with some sort of goo/rtv/sealant. Apparently, over time that material degraded and flowed into the mesh, giving the vent opening a serious case of atherosclerosis! This problem wasn’t easily visible when the screen was installed over the dark tube interior, but it was easy enough to see when held up to the light (see attached photo).

Lessons:
1. Innovations to increase safety can create their own problems.
2. Inspect vents carefully, preferably with a flashlight.

 

[D-Dubya]

Wow. I can't even say that word, much less spell it...

But a good find. On a plane or a person.

 

[BobTurner]

I noticed one of my fuel tanks was gurgling quite a bit and filling slowly. I suspected a blocked vent, but ….

I am puzzled that a blocked vent would lead to ‘filling slowly’. The open gas cap area is huge compared to the vent size, the air should easily exit there. ??

 

[Dugaru]

I am puzzled that a blocked vent would lead to ‘filling slowly’. The open gas cap area is huge compared to the vent size, the air should easily exit there. ??

I can’t fault your physics. All I can say is that it gurgled a lot and you had to top it off slowly or you wouldn’t fill it. Still gurgles but seems to fill faster now. Perhaps not? I suppose it’s possible I serendipitously found a potentially dangerous condition after misdiagnosing the actual problem.

 

[rv6n6r]

I had the little screens on there originally but took them off as the vent hole is barely larger than the screen gauge, plus it's difficult to proseal it on without having the goo obstruct (possibly what happened here). One data point anyway.

 

[rvbuilder2002]

I am puzzled that a blocked vent would lead to ‘filling slowly’. The open gas cap area is huge compared to the vent size, the air should easily exit there. ??

It won’t.
The tank vent is only relevant when the fuel cap is in place.

 

[cgeyman]

the slow fill and gurgling is the time it takes for fuel to flow in and around the baffle system. normal

 

[Dugaru]

the slow fill and gurgling is the time it takes for fuel to flow in and around the baffle system. normal

One tank seems worse than the other. If a line guy tops it off without waiting, it will be shorted by almost 3 gallons. Normal variation?

 

[cgeyman]

Since I am my own line guy, I can put in 1 to 1 and 1/2 more gallons if I give it time and the plane is on pretty level surface. But it takes 2 minutes to fully settle, though I haven’t ever timed it.

 

[Dugaru]

I’ve done a bit more experimenting and observing, and the difference with the vent opening cleared is that the audible gurgling now continues after the fuel cap is replaced. Never heard that previously.

 

[Stevea]

One tank seems worse than the other. If a line guy tops it off without waiting, it will be shorted by almost 3 gallons. Normal variation?

Yea, because they are in a hurry and won't wait for the tank to settle.

Since I am my own line guy, I can put in 1 to 1 and 1/2 more gallons if I give it time and the plane is on pretty level surface. But it takes 2 minutes to fully settle, though I haven’t ever timed it.

I'm my own line guy too. I stop about 1/2" down from the tank filler cap flange.....and watch the fuel level drop down a bit...then top off. Takes maybe 5 - 10 seconds.

If anyone's tank takes "minutes" to settle, I am going to suggest the fill / vent holes between tank ribs (NOT vent line or the holes where the vent line runs through...those are entirely different) were not drilled during the build, or they are undersized from what is called out on the plans. Or, they ended up being partly or totally covered by proseal.

 

[bjdecker]

Trying to get my head around this one -- I am almost 100% certain the vent being clogged isn't the source of the low/uneven fill rates and the gurgling. Perhaps there's something deeper in the tank amiss -- rib/baffling holes being plugged with debris?

Run a borescope into the tank and see what those baffle holes look like.