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Why is my tank pressurizing and NOT venting?

Omega232Devils

Well Known Member
I have searched the forums and found some good information, but unfortunately has not corrected my problem. Any additional help is appreciated.

I have been plagued with a tank leak in my quick build tank. (Left one only) I removed the tank about two months ago and did a complete resealing of the inboard rib section that I could get to through the access panel. I was able to clean, prep and re-proseal the inside and outside of the tank. I pressure tested the tank for 36 hours with no leaks (1psi Balloon). Filled the tank with gas and let it sit for 48 hours, no leaks. The tank flew fine for about 2 weeks with no issues. In the meantime I started noticing that my left tank would build up pressure, both in flight and sitting (temperature changes most likely). I found this when I popped the fuel cap and could heard the pressure releasing.
After the two weeks, my tank began developing a small leak in the lower corner closest to the fuselage. (Same spot as the first leak) I pulled the tank off yesterday to do the repair and did the baloon pressure test and soapy water. After 6 hours with pressure, there was no evidence of a leak. (Frustrating) After I cleaned the old proseal off, and prepped the surface for new seal, the balloon was still inflated. When I put the new proseal on, I noticed a small air bubble between the 2nd and 3rd rivet from the back. (I have to admit, I didn't catch it right away, as I just kept adding more proseal trying to cover up the bubble and it kept coming back):confused: The light bulb finally came on, that this is the source of my leak and explains why my tank leaks when I forget to pop the fuel caps and relieve the pressure.
So that is the good news, I have identified my leak and when it is leaking. (Under pressure = leak / No pressure = No leak) So I looked to my vent system. I have never had fuel come out of the vent ports, regardless of level of fuel. I was able to take my air hose and blow air through the vent on the bottom of the fuselage and feel it on the connector going to the wing tank. I was able to blow air through the vent tube on the tank and feel with my finger the air coming out of the tube by the fuel cap. So my vent lines are clear.
So I can seal the leak pretty easily, but until I identify and correct why my tank is pressurizing and not venting, my leak is going to continue to return.

Where do I go from here?

Dan
 
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I was able to take my air hose and blow air through the vent on the bottom of the fuselage and feel it on the connector going to the wing tank. I was about to blow air through the vent tube on the tank and feel with my finger the air coming out of the tube by the fuel cap. So my vent lines are clear.

Dan

Do you mean able, instead of about??

Following is assuming you meant "able"

I can only think of a couple things that could be causing this, one is fuel trapped inside the low point of the vent, other is if a blob of sealant is sitting on the inner tank end of the vent line, and is acting like a one way valve, like a flapper.

Try to suck on the vent line, while it is hooked to the tank, with the cap off----if you can not get air to come out the vent line, but you do get air to go in when you blow on it, then I would look real close for the blob I mentioned above.

I had this happen to one of my tanks, otherwise it would never have occurred to me to even think of such a thing.
 
test the vent

Remove the gas cap and try to blow through the vent tube to see if it's clear.
 
So that is the good news, I have identified my leak and when it is leaking. (Under pressure = leak / No pressure = No leak) So I looked to my vent system. I have never had fuel come out of the vent ports, regardless of level of fuel. I was able to take my air hose and blow air through the vent on the bottom of the fuselage and feel it on the connector going to the wing tank. I was about to blow air through the vent tube on the tank and feel with my finger the air coming out of the tube by the fuel cap. So my vent lines are clear.
So I can seal the leak pretty easily, but until I identify and correct why my tank is pressurizing and not venting, my leak is going to continue to return.

Where do I go from here?

Dan

So did you do this? You didn't say. Just connect a rubber hose to the vent fitting on the tank or the belly and blow through it. Sounds like the vent line in the tank is clogged. If the vents are clear, the tank will not pressurize.
 
Correction

Thanks guys. Man, one word really can change the meaning of a paragraph huh? haha


I did blow air through the vent system, both the fuselage portion and the tank portion. (The tank is off the plane for the repair.) How much pressure can I put through the tube in an attempt to 'dislodge' any material that may be acting as the one-way valve?

The tank tube is not totally block, because I am able to inflate the balloon during my pressure tests.

Thanks again,

Dan
 
Make sure the cap is off

If the cap is off you can hit it really good. 3003 tube can take a good deal of air pressure. Last tank I looked at was pressure tested to only 3 psi and leak tested and 1.5psi so double check that the cap is off the tank!
 
leaking vent line inside of tank

You might have a leaking vent line inside of tank at the fitting. Have found this before and when fuel is over the fitting fuel it will get into the vent line and cause pressure to build in the tank. Usually burping out fuel with full tank. Can be found by air testing vent lines.
Squeak
 
Now what?

Ok I just put 80-100psi through the vent lines. The fuselage was clear, strong pressure out. Vent line on tank with fuel cap and drain valve removed air passes through. I cant measure exactly, because of vent line location in tank, but appears to not be blocked.

Some have mentioned blowing from fuel cap side, but how did you do this? My vent line is straight on bulkhead and not bent toward the cap.

I have not pulled the access panel yet to look at inside fitting on vent line. Since there has never been fuel out the vent drain could a hole here even be an issue? My tank is pressurizing and the fuel is not venting, but rather making its way out through a hole created by the pressure inside?

Any other thoughts?

Thanks guys.

Dan
 
Get a helper to run the blow gun, get a flashlight, and an inspection mirror.

Take the fuel cap off, insert the mirror and using the flashlight, find the end of the vent line.

Have your helper blow pressure into the vent line from the other end of the tank.

Look for a blob of sealer acting as a one way valve on the inner end of the vent line.
 
all clear

Mike,
Did the mirror check and no evidence of proseal valve. Good airflow.

Any other ideas? I am stumped.

Dan

UPDATE: I put fuel cap back on, installed Vans fuel tester valve in drain port. Covered fuel line and left vent line OPEN. I slowly put air in tank using very little pressure and felt for air coming out vent. Air flowed with increase of air going in. I.e. when I increase inflow air pressure, exit flow increased. So doesn't appear to be blockage or I have removed it somehow.

Any other suggestions or tests?

Dan
 
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discard the vent line leak

If you have no fuel coming out line with pressure in tank than discard the vent line leak inside tank. That pressure should not be building up with vent lines open.
Squeak
 
Go back to the balloon

How about this?

Do the standard pressure check with a balloon. Remove the balloon then open the fuel cap a few seconds later to see if there is still captive pressure.
 
If your tank is pressuring while in level flight it is most likely not a blocked fuel vent. A blocked fuel vent will typically:

a) stop the engine while running on that tank due to the inability of the fuel pump(s) to pull fuel through the system.

and/or..

b) Cause the tank to collapse when descending from serious altitude.

Did you try blowing air both ways? There could be FOD in the vent line acting as a flapper valve. Hey, it's worth a shot.
 
Mike,

UPDATE: I put fuel cap back on, installed Vans fuel tester valve in drain port. Covered fuel line and left vent line OPEN. I slowly put air in tank using very little pressure and felt for air coming out vent. Air flowed with increase of air going in. I.e. when I increase inflow air pressure, exit flow increased. So doesn't appear to be blockage or I have removed it somehow.

Any other suggestions or tests?

Dan

O.K., so the vent line inside the tank is not the problem.

What about the vent line that is not inside the tank??? I am referring to the part inside the fuse here.

Check it for free air motion both in, and out.
 
great help guys! thanks

Mike,
The fuselage lines are easy to check and are clear. 100 psi pushed through them both directions.

I have pushed 100psi from vent opening on tank where it connects to fuselage, through tank vent line, exiting the vent line inside tank near fuel cap. All is clear there.

Because I cant get to the vent line inside the tank close to the fuel cap to put 100psi back through the vent line to the outside connector, I had to do best I could and basically do pressure test with NOTHING on the vent line and push air through valve steam installed in quick drain port. Max of 20 psi and hear and feel air coming out.

Just put another balloon on and put air in tank. Balloon fills just like it always does. A little air in balloon at start of air input, then stops for a few seconds while air is being put in,then fills up rather quickly.

Good info on pressurizing in level flight, but what would cause the pressure being released by opening fuel cap after landing?

Dan
 
Sure wish you lived closer, this is just not something that is going to be easy to unravel without being hands/eyes on the plane.

Going back to your first post, you said "The tank flew fine for about 2 weeks with no issues. In the meantime I started noticing that my left tank would build up pressure, both in flight and sitting (temperature changes most likely). I found this when I popped the fuel cap and could heard the pressure releasing."

Something HAS to be keeping the pressure in the tank, and it would appear that that something occurred after flying for about 2 weeks.

Did you do any inverted stuff in that two weeks???

Wash the plane, or fly through rain?

Hit a lot of bugs??

Describe how the vent line runs in the fuse, or better still, post a photo.
 
Are you doing the test with fuel in the tanks?
There is an outside chance that the fuel bay that the vent line is in is somehow sealed from the other bays at the top of the rib. If the tank was empty, or low enough to uncover the lower rib/baffle holes, it would not build any pressure. However, if the fuel level was such that the baffle holes where covered up and that last bay with the vent line in it was sealed from the other bays, this could possibly happen. I hope that makes sense.
This is a real long shot I will admit.
(sorry MIke, stepped on your post while I was typing mine.)
 
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Something not right here!

Obviously your airplane doesn't understand physics.
How is it possible to build pressure in a tank that has an open/clear vent line?
 
Mike,
Yeah this would be a lot easier in person. I did mild aerobatics, no over the top, and did wash the plane. My fuselage vents are cut at 45 angle and the line enters fuselage by wing goes up along fuselage about 24" high, 90deg turn and runs forward to fuselage, another 90 deg turn and runs down about 24" to fuselage floor where it is connected to outside vent.

Tank has one 90 deg turn before it enters tank, then runs straight along front of tank.

Will try to post picture. I wish there was a way to test pressure differential, so if I put 3 psi into the tank that 3 psi came out (or something close ).
 
Fuel level

The 24" rise in the vent inside the fuselage can cause a .8 PSI pressure in the tank if filled with fuel. Sorry if I missed the fuel level earlier. If the level is above the vent inside the tank then the vent will get fuel in it. It requires 1 PSI to raise water 27". Fuel is lighter than water and you said you had a rise of 24" so your tank would have about .8 PSI in it if the lfuel level was above the vent or if the vent had fuel in it.
 
What Mark said above. My RV7A often has pressure in the tanks that relieves when the fuel caps are opened. However, if the airplane sits in the sun/warm hangar this pressure gets high enough that it blows the fluid in the vent line out - fuel can then squirt from the vent a couple of feet. I've learned to avoid being in front of the vent outlets without the fuel caps loosened. One unexpected, forceful and sudden faceful of gas is enough to have taught me that lesson!

One of my QB tanks had an issue where the upper skin-to-outboard baffle had too little Proseal on the inside joint. I used Rocket-like fiberglass wing intersection fairings, and when the fairing was screwed to the flange it exerted upward pressure on this joint, causing a leak. It only appeared 1). with the fairing on and 2). with the tank full. It was frustrating to find and wasn't a lot of fun to fix - I had to remove the tank, remove the access plate, and re seal the joint working through the hole.
 
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Exit vent
IMG_20120119_140153.jpg


Inside from vent
IMG_20120119_140407.jpg


Along top inside
IMG_20120119_140456.jpg


Out to wing
IMG_20120119_140521.jpg


Left side of fuselage
IMG_20120119_140138.jpg


Hope this works. Sorry forcrappy pictures I am working with only my phone in the hangar.

NOTE: I am pretty confident that it is not the vent lines running through fuselage as I have been able to put high psi through both directions.
 
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My right tank will do this just a bit. The vent line inside the tank is below the fuel level when the tank if full dammit. That said, on a -7, the vent line goes up about 16 inches before it turns down. So, it I overfill the tank, I should expect to find 16" of pressure on it after it heats up and expands.
See if you can feel the end of the vent line. Does go all the way to the top of the tank or is it an inch or so from the top of the filler neck?
 
Update!

Ok guys here is where I am and where I left the tank. First, thanks very much for all comments. I hope i answered each one.

JD, I tried the balloon release then pull the cap and there was no pressure built in the tank.

Jamie, great info. Ihave not had my engine stop, nor tank colapse during descent so that is good.

The tank is sealed up again on outside where I saw the bubbles. I pressure tested the tank and the balloon is holding steady. So i feel confident that i only have a leak when the tank is under higher pressure.

My plan is to install the tank on Saturday giving the ProSeal 4 days to cure! (not ideal, but I am sure I am going to have to remove access cover and seal from the inside again. I have ordered a new gasket from Vans, so in the meantime I would like to see if the tank is still building pressure. ) After I hang the tank I will fly and experiment with different fuel levels and see if the tank continues to fail to vent when parked and left overnight.

Thoughts? Any recommendations on checking the tank?

Thanks guys!
 
See if you can feel the end of the vent line. Does go all the way to the top of the tank or is it an inch or so from the top of the filler neck?

The vent line goes straight through the tank, no bends. It is high up on the tank, but I dont think it is quite as high as the filler neck. When tank is full, the vent is under fuel level. I have heard of guys bending the vent line so it ends `at` the fuel filler neck!

Dan
 
so in the meantime I would like to see if the tank is still building pressure.

The tank will build pressure, and that is normal. The forward facing slant cut on the vent fitting will cause pressure to build, and so will the fuel heating up after it is put into the tank.

After I hang the tank I will fly and experiment with different fuel levels and see if the tank continues to fail to vent when parked and left overnight.


Thanks guys!

Now you are starting to define the problem correctly------it is a failure to vent that is causing your problem.

The head pressure of a vent line with fluid in it is pretty small, but if it is greater than the pressure a sub standard leaky rivet seal can hold, guess where the pressure is going to go??

This is why I was asking earlier about a flap/blob of sealant that could have been acting as a one way check valve in your vent line.

By the way, one of your photos shows what sure does look like a crack in the skin, the one with the vent fitting on the bottom of the plane. You need to deal with that, if it is in fact a crack.

IMG_20120119_140153.jpg
 
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Paint build up

Haha Mike. Like they say a picture adds 10lbs to a person i guess it adds one more problem to planes. Haha.

After I uploaded that photo i saw the same thing and let out a few choice words then got on my back to take a look. Luckily it is not a crack, it is paint built up that looks like some fuel or oil got to and has loosened it up with the help of the windstream!

Good looking out Mike. I would rather someone point it out then me continue to fly with potential problem.

Dan
 
I see that the under belly vent is open and could be plugged by insects or other means. I have window screen safety wired to mine to prevent that from happening.
 
....How is it possible to build pressure in a tank that has an open/clear vent line?

......Fuel is lighter than water and you said you had a rise of 24" so your tank would have about .8 PSI in it if the lfuel level was above the vent or if the vent had fuel in it.

....My RV7A often has pressure in the tanks that relieves when the fuel caps are opened.....

Mark's explanation of why fuel tanks known to have clear and unobstructed air vents can build a slight amount of pressure makes perfect sense to me because I have often noted the same phenomenon. In fact, that natural build up of pressure is one of the elements I raised as an aggravating issue in the famous paint blister thread back in July of 2009:

http://www.vansairforce.com/community/showpost.php?p=341469&postcount=105

2crm2rq.jpg

Surely with thousands of RV's flying, other builders have noted the exact same thing when air rushes out the moment they crack the gas cap loose. Then again, I'm sure some builders adjust the fuel cap mechanism so loose that pressure does not have an opportunity to build within the fuel tank.
 
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Then again, I'm sure some builders adjust the fuel cap mechanism so loose that pressure does not have an opportunity to build within the fuel tank.


Good point Rick. This brings another question, how tight of a seal should the fuel cap provide? (Now please, before those with no issues with their tanks venting or pressurizing provide non-constructive feedback, just humor us a moment) Has anyone tweaked the fuel cap fitting to provide another way to 'vent' the tank but tight enough to not allow the airstream to create a vacuum and suck fuel out?

Even with clear vent lines, I have a feeling that my left tank vent system is not adequate enough and Mike hit the nail right on the head, that as my tank builds pressure, I obviously have some weaker points in my tank that allows the pressure AND FUEL to escape, before then vent system is allowed to work.

Dan
 
I would have to go with Mel on this. It is not a complicated system. It is a tank with a vent line. If built properly, the only pressure that would ever be on the tank would be the head pressure of any gasoline trapped in the verticle section of the vent line....and that ain't much.......I would not loosen the gas caps enough to vent any kind of pressure as that could introduce moisture into the tanks if caught in the rain. I think you need to recheck that vent line.

Ed Booth, RV-6, 7, 9A, & 10
 
Agree

... It is not a complicated system. ... If built properly, the only pressure that would ever be on the tank would be the head pressure of any gasoline trapped in the verticle section of the vent line....and that ain't much.......I would not loosen the gas caps enough to vent any kind of pressure as that could introduce moisture into the tanks if caught in the rain. Ed Booth

Great point Ed. I agree with you, so just playing devils advocate to learn more about my system and what others are doing. (Just to reiterate, these are QB tanks, I did not build them so they are built as properly as the factory does.) I know the system works, as my right tank works great. My left one does not however and all the normal checks are passing, so something is up. Hence the outside the box thinking here. Like I said, I am confident the problem does not lie with my fuselage piping, but rather the piping in the tank.

Note: : I am not looking for a 'shortcut', rather I am looking to 'do the right fix'. The leak is more of an annoyance than it is an issue. The amount of fuel leaking is minimal, but is a pain to see when checking fuel on pre-flight.
I am wondering how difficult it would be to replace the vent line inside the tank. Other than the pain of fishing a 4' piece of tubing through the ribs without being able to see the holes. Well I guess I could get a boroscope to help feed the tube down. Has anyone done this before? Is it difficult? I think if it is not too difficult, since I have to remove the access panel anyway, might be best to put in a new vent tube.

Keep the comments coming.

Thanks,

Dan
 
have you tried snaking the vent line in the tank?

Try using some heavy nylon line used on the spool of a weed wacker and see if you can snake it through the line easily.
 
....I know the system works, as my right tank works great. My left one does not however and all the normal checks are passing, so something is up....
Dan,

You seem eager to follow through on this vexing problem using whatever resources necessary. I admire your thoughtful approach and would really like you to resolve this if for no other reason than enhancing the wealth of knowledge stored in VAF archives. This is an issue I don't ever remember being raised before. The fact that I and others have occasionally experienced the pressurized phenomena when everything about the system is built exactly per plans, makes me even more interested.

Now if someone can just step forward with an efficient home handyman way of accurately determining once and for all that the vent line installed in the QB tank is fully open and working properly.
 
snake

Try using some heavy nylon line used on the spool of a weed wacker and see if you can snake it through the line easily.

I found a spool of 'weed eater' heavy duty line does a good job snaking these pipes. Stiff and thick enough to be effective.
 
good timing

I have tried heavy safety wire, but didn't work well. I am in the aviation section of Lowes hardware now so I will pick up some weed eater string.

Will snake the tubes and report back. Thanks guys.
 
I used some cable, similar to the rudder cable.

I suspect speedometer cable would do a good job also.

Chuck it into a drill, spin slowly ---- helps go around corners.
 
The vent line inside the tank does not have any sharp bends. Stiff piano wire would be a good choice to run through it. Normally the end by the filler neck is held in place by a clamp rivited with one of the filler neck rivits. Good luck.

Ed Booth, RV6,7,9A & 10
 
....Normally the end by the filler neck is held in place by a clamp rivited with one of the filler neck rivits. Good luck. Ed Booth, RV6,7,9A & 10
Ed brings up a good point Dave. Since you did not build the fuel tanks, you might find this image inside an -8 tank helpful. Try inserting an inspection mirror down the filler flange to verify positive clearance between the end of the vent tube and the outboard rib. You should see something like this. If possible, try to maneuver the mirror so you can inspect the open end of the vent tube to make certain no proseal is obstructing it. I don't know doing that is possible, but maybe.

j9x3ma.jpg


BTW, while you are looking inside that fuel tank, also verify if the shop heads of the rivets you can see are encapsulated or not and let us know what you find.
 
BTW, while you are looking inside that fuel tank, also verify if the shop heads of the rivets you can see are encapsulated or not and let us know what you find.

I have used a mirror before and the vent tube does have clearance at the end of the rib and i was able to verify no ProSeal was on the end of the tube.

I will take a look at the shop head rivets tomorrow.

The line inside of the tank does not have any bends in it, but the outside connection does have 90 degree bend. So if the wire snake does not work i will remove the connector and snake the tube from there. Going to be a busy day tomorrow.

Thanks guys!

Dan
 
A friend of mine has a 6-a w quick build wings,the b nuts on the vent line were never tightened up on the vent line in the tank,the vent line in the fuselage would fill with fuel and be purged out as the fuel in the tank expanded,or when he landed he could pull the cap off to relieve the pressure.Hope this explanation makes sense,after tightening the b nut in the tank, no more problem.
 
A friend of mine has a 6-a w quick build wings,the b nuts on the vent line were never tightened up on the vent line in the tank,the vent line in the fuselage would fill with fuel and be purged out as the fuel in the tank expanded,or when he landed he could pull the cap off to relieve the pressure.Hope this explanation makes sense,after tightening the b nut in the tank, no more problem.

This sound like a good thing to look at, is sure would cause the problem that Dan is having.
 
confused?

Bob/Mike,
I am confused on the B nut post. I will check the connections when I pull the access panel, but my tank is not venting fuel overboard through the vent lines. If it were, then the tank would be working as advertised. Can you explain this another way? Is this something that would be evident from the outside of the tank? Maybe by shaking the vent connection on the tank?
Sorry i wasn't able to make it to the hangar today, it was a rare sight in San Diego as it was raining all day so I stayed home and built some work benches for the hangar! Will be there tomorrow.

Dan
 
If it Ain't Pressurization, what is It?

260643b.jpg

Some people just don’t buy into the idea that a properly vented fuel tank can, to some extent, pressurize. I’ll show why I think pressurization routinely exists and I’ll do that by laying out my own experience. Greensfield (M71) where I am based is a small semi-private airport. The owner, a wealthy self-made man and acquaintance I tend to keep at a friendly distance takes a lot of pride in his little airport and he notices everything that goes on. For all his many interests, M71 is his true pride and joy. The 3200' runway, the ramps and the apron in front of my hangar are topped with asphalt. I'll bet some of you already know where I am going with this, but let’s not get ahead of ourselves. With no facilities, every tenant at Greensfield is on their own as far as fuel is concerned and when I need fuel, I routinely fly to KSET, 25NM distant. To me, upon taking the active runway at Greensfield for departure, it is a point of pride to make that leg to KSET in under 10 minutes and certainly no longer than 12 minutes. Believe me, for guys flying spam cans out of Greensfield, that kind of time is amazing. Anyway, upon arrival at KSET I land, turn off its nice 4000’ runway and head straight for the CAF’s drive-up self-service 100LL pump. With the tanks topped off, I depart and usually fly straight back to M71 and park the plane in front of my hangar. Like a lot of RV builder/pilots, upon exiting the plane I may walk around it a bit, wipe the bugs off the leading edge and clean the canopy, slider of course. :) Sometimes, after all those chores are done, I just retreat into the shade of my hangar and sit down in a chair endlessly admiring the creation I still can't believe I built. With my dog and constant companion Bubba present, Darla! sits outside shining in the sun. I may be drinking a cup of coffee while the morning passes. The sun rises higher and the air warms up. The airport regulars are up and about, friends may drop by or I may go visiting at another hangar. In 2005, my recently completed RV-6A was still in Phase One when I had an eye opening experience that taught me a little something about fuel tank pressurization. One day, I was walking back to my hangar after visiting others. As I rounded the corner of the hangar, I caught first sight of Darla! as expected. She stood there posing just as proud as ever only this time was relieving herself with an almost steady stream of fuel from the topped off tanks onto that soft asphalt below. By the time I noticed the damage there was a significant hole already burned into the asphalt thus becoming a topic of conversation with the airport owner I really didn’t want to have.

Now for the fuel to have done that, it first had reach the level of the vent tube inside the fuel tank, enter it, travel inboard toward the fuselage, make a couple of right angle turns, then climb a vertical length of tubing embedded in the fuselage that tops off several inches ABOVE the level of the fuel tanks affixed to the wings. Now what could possibly cause that fuel to rise? Even if the fuel does not make it to the top of the vertical portion of the vent tube and just sits in there midway, it sure seems reasonable to me that any rise in the fuel's level must be caused by the forces of expansion and pressure. However slight that expansion and pressure may be, it is almost certainly exerting some measurable force (however slight) against every piece of structure and rivet attached to that fuel tank.

35jxo9y.jpg
 
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Rick, I don't think anyone ever said that the tank can't "pressurize" and force fuel overboard. This happens whenever the aircraft is fueled and placed in the sun, thereby heating the fuel until it expands and builds pressure. The original problem stated that the tank pressurized and did not force fuel overboard even though the vent line is open and clear.
 
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Obviously your airplane doesn't understand physics.
How is it possible to build pressure in a tank that has an open/clear vent line?

Point taken Mel. Your prior post seem to convey the idea that it is impossible for a tank with an open/clear vent line to build pressure at all. I believe that pressure generated inside a fuel tank has the potential to aggravate other vexing issues, even rivet locations that do not normally come into direct contact with raw fuel.

29mopxj.jpg

But you are entirely right Mel. My thoughts would have been better suited for insertion in another long running thread.
 
Your prior post seem to convey the idea that it is impossible for a tank with an open/clear vent line to build pressure at all.

You're right. I should have been more explicit to mean that the tank should not be able to build "significant" pressure without dumping fuel overboard.
 
tank pressure

As I stated in an earlier post, and agreeing and expanding on Mels comments, The tank if built correctly, can only contain enough pressure that would be equal to the height of a column of fuel that may be pushed up in the verticle section of the vent line (inside the fuselage). Any pressure greater than this would push fuel out of the overflow fitting thus limiting the amount of pressure that could be held in the tank. If you have pressure build up greater than this, there is a restriction somewhere in the vent line.


Ed Booth, RV6,7,9A & 10
 
difference between leftand right tanks

Could the problem really be that the cap on the right tank does not seal and vents any pressure caused by gas filling the vertical rise in the vent lines? That would explain the difference between the two tanks and you may be looking at the wrong tank (other than resolving the leak issue) to explain what you are seeing.
 
Update!

Ok guys, sorry it has taken me so long to write back with an update, but I have been elbow deep in cleaning up pro-seal.

I removed the access panel today to take a look at the vent line. I tried to snake the tank vent line using the recommended weed eater line, but couldn't get the line through the 90 deg AN fitting on the outside of the tank.
IMG_20120122_184103.jpg

Inside the tank looked like this:
IMG_20120122_184132.jpg


So I removed the proseal around the connection and took apart the vent line so I could snake the vent tube. The string did not easily go all the way through the vent tube, but took some TLC. I then hooked it to my drill and snaked the tube several times. There was no evidence of blockage on the string when I pulled it out, but it did go in a lot easier the second and third time. (So fingers crossed this worked.)
IMG_20120122_213044.jpg

IMG_20120122_213004.jpg


I prepped the inside to proseal where the leak was originating from and will seal everything up tomorrow, pressure test on Wednesday, leak check with fuel on Thursday and hope to be flying by this weekend. I will keep this thread updated in case anyone in the future has this issue.

Thanks for all the posts.

Dan

P.S. I did snake the fuselage vent tube and all was clear.

The fuel cap on my right tank is brand new. The left fuel cap (trouble tank) is the original cap in good condition and no evidence of anything that would warrant a replacement.
 
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