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Why is my tank pressurizing and NOT venting?

UPDATE!!

Ok time for an update. I mounted the tank after sealing the leak and snaking the vent lines. Filled the tank and went for an hour flight today. After landing I popped the fuel cap and there was NO pressure release. YES! Well this is just half the battle. I still need to verify my tank will vent fuel over when fuel vapors pressurize the tank on a hot day.

Will post another update after a few weeks of testing. Thanks to all who helped!

Dan


MODERATORS: Recommend changing thread title to more accurate title such as "Why is my tank pressurizing and NOT venting?"
 
Something Else to Consider...

Dan:
Pressure is working two ways here and the ultimate cure is a free-flowing vent line.
Sitting on the ground and in the sunlight(as Rick suggested), the fuel and vapor above it heats and expands. If the tank is full enough, the result can be fuel being pushed out the vent line, or blown out the cap when it's loosened. In this case, the tank was pressurized.
Flying the aircraft with a blocked or partially blocked vent can lead to much lower pressure inside the tank due to the demand of the fuel pump. As mentioned, in the extreme, this can cause the tank to collapse. This will also lead to a venting sound when you land and release the cap. Here, the tank is de-pressurized.
Same fix for both problems. Sounds like you're there.
Terry, CFI
RV9A N323TP
 
I have a friend witha quick build 6,he had problems with pressure on the cap every time he opened the cap after it sat for a few minutes on the ground,ended up having the b nut loose on the vent line inside the tank,fuel would seep into the vent line effectively sealing it off then as the sun warmed the tank it built pressre and since fuel was already in the vent it pushed it out.took the acces plate off and tightened and prosealed the nut and no more problem,called vans and they said your supposed to ck that,i thought that was the point of the quick build tanks,they should have done it.....
 
Inspect the tank for clear protective film

There's a SB concerning plastic protective film being mistakenly left on in the tank during construction. This film is blue but before 2000 it was clear and hard to see thereby possibly being left on an interior wing skin somewhere. If so, it would conceivably come off eventually. If that got itself wrapped around the pickup tube, there could be a bad day a comin. If it floated up to the vent tube near the filler hole, it could act like a one way valve periodically with the symptoms decribed in this thread. My thoughts anyway. Something to look for.

see...

http://www.vansaircraft.com/pdf/sb04-2-1.pdf

Bevan
 
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Good post Bevan, I too have seen clear cigaret plastic in a diesel tank......that took forever to figure out why the engine would starve for fuel every now and then. What about decreasing the head of fuel, I am doing the three coil loop to the vent in the wing gap fairing.
 
opposite problem

This was an instructive thread, but I am having the opposite problem-- my right tank vents fuel, even when the tank is nowhere near full, and in the hangar out of the sun, even in cool weather. I also have noted a "whoof" of pressure when I open the tank after it has been sitting for awhile... not sure if it is positive or negative pressure. I did manage to blow into the fuse vent with a piece of tubing and I heard the line venting in the tank. I also inspected the vent opening by the filler cap with a mirror and did not find anything unusual.

I noted the reference to the "b nut" in the thread. I could not find that in the plans.

Any ideas what to check next?
 
Informative thread! I have been trying to debug my own left tank pressure problem. And at first glance, my vent lines seem clear. It was good to read the scenario of fuel filling the vent line and allowing some pressure.

A question or two if I may.

My tanks are slow built and the vent inside the tank terminates next to the fuel fill neck, and pretty close to the inside of the top skin. I generally leave my tanks about an 1/8" below the fill neck when I top off. I am wondering how much fuel can expand, and what is a safe level for filling the tanks. OR, if you have to leave the tanks significantly lower, does anyone know how much fuel we would be leaving behind? I always set my fuel computer to 38 gallons after filling to below the fill neck as I described, figuring that was a conservative estimate of the actual level of fuel.

The times I've landed with a full tank on the problem side of the plane, or come back to the hangar a week after topping off (and in warmer temps), the fuel spits out the fill neck as I release the cap. I have a dirty pavement floor in the hangar and it is hard to tell on any given day if I leaked any fluid. I have recently begun putting clean paper towels under the vent to see if I could detect fresh fuel or dye.

Thanks. Andy
 
edneff said:
...

I noted the reference to the "b nut" in the thread. I could not find that in the plans.

Any ideas what to check next?
Click to expand...

If you suspect a venting problem, the B-nut in question is at the inboard end of the vent line where it exits the tank. Fill the tank enough for the inboard end of the vent line to be submerged, then blow into the external vent port. If you hear bubbling instead of venting, a loose B-nut is the problem.
 
longranger said:
If you suspect a venting problem, the B-nut in question is at the inboard end of the vent line where it exits the tank. Fill the tank enough for the inboard end of the vent line to be submerged, then blow into the external vent port. If you hear bubbling instead of venting, a loose B-nut is the problem.
Click to expand...

I heard bubbling on first puff, but then it stopped, so I am guessing that there was fuel in the vent line that bubbled out... To be sure I will take a longer blow next time at the hangar and see if the bubbling persists...
Thanks
 
So I am pretty sure that the vent line "B" nut through-tank fitting on the inboard side of the fuel tank is not tight, and that's what's causing the venting even when the tank is not full.

Question... To tighten that fitting, can I reach it through the round access panel on the side of the fuel tank? I don't have to remove the fuel tank to do that do I?

Any tips or hints on accomplishing this?

Tks.
 
I had the same issue. I noticed a fuel stain appearing when when the plane was out in the sun and a while later a shot of fuel came out of the vent. I had to remove the tank to re seal it so I tightened the nut at the same time. I think depending on how skinny your arms are you may be able to do it without removing the tank. It's worth a shot. If you can't, it's not a horrible job to remove the tank. It took me about an hour to remove it and the same to put it back on.
 
Any hints or tips for removing the RV7 fuel tank???

So it's condition inspection time, and I attempted to remove the T-708 access panel on my right tank so I could access and tighten the "B nut"...

OMG. I used a sharpened putty knife and then a steak knife and a few other choice tools, but I could not budge that access panel. It's as tight as a tick. I am afraid that I am going to destroy it getting it off. I asked my A&P about it and he said the black adhesive on the panel (pro-seal) is intended to be permanent adhesive, e.g., for the inside of the tank, and that he doubts I will be able to remove it easily or without damage... He suggests removing the tank from the wing and then going at the access panel with a newly ordered access cover on hand ready to go once the old one comes off and the B nut is tightened.

He also says that I should use a less permanent adhesive for remounting the access panel cover so I don't have to go through the same drill again if I have to remove it in the future... he named a sealant, but I don't recall what it was, it might have been pink (?)

So I did not build this aircraft and it will be my first time removing the tank...

Any hints?

There are seven "Z brackets" (T-712 tank spacers) with three bolts each going through the spar web, but it looks to me like only two of the brackets are accessible from the access panels under the wing... I guess I will disconnect the fuel and vent lines, then the Z bracket bolts, then the screws on the underside, then the topside screws.

Thanks for your suggestions.
 
edneff said:
There are seven "Z brackets" (T-712 tank spacers) with three bolts each going through the spar web, but it looks to me like only two of the brackets are accessible from the access panels under the wing...
Click to expand...

Nope, you can get to them all through the access panels (otherwise, how would you put the tank ON the wing in the first place?). Probably have to disconnect the pushrod to the aileron bellcrank first, though (of course, you'll have to do that to take the tank off, anyway). But you can reach them all through the access holes.
 
Use a piece of sheet metal to make a 2" X 10" scraper blade. If you lubricate it with MEK and slowly work it into the joint (Keep it wet with MEK). It will very slowly cut through the proseal. Round the edges on the scraper and use care not to gouge into the rib. - 20min and you will have the cover off. Then use MEQ and scotch bright to remove all of the material.
 
Yes, my hint is to leave that perfectly well sealed panel alone. What makes you think the nut needs tightening? This is not a standard condition inspection item.
What is pretty standard is the use of proseal in this area.
Leave well enough alone and go fly your plane instead of removing the tanks

Erich
 
pa38112 said:
Use a piece of sheet metal to make a 2" X 10" scraper blade. If you lubricate it with MEK and slowly work it into the joint (Keep it wet with MEK).
Click to expand...

I am concerned about using the MEK on the fuel tank because I'm thinking it might weaken the bonding around the nutplates for the access panel, or get inside the tank and dissolve some of the pro seal... Am I just being paranoid? Don't want to fix one problem and have another one.
 
erich weaver said:
Yes, my hint is to leave that perfectly well sealed panel alone. What makes you think the nut needs tightening? This is not a standard condition inspection item.
What is pretty standard is the use of proseal in this area.
Leave well enough alone and go fly your plane instead of removing the tanks
Click to expand...

Yeah, I totally agree Erich... Problem is that I share a hangar with my A&P, and he has observed the tank venting on more than one occasion when the A/C was put away with tanks over half full. I am 99% sure that the cause is the vent line B nut on the inboard rib is loose, because it stops venting when the tank fuel is below the level of the B nut. (about half full)

A&P says he won't sign it off with a "leaking" tank, even if it's the vent line leaking. I offered to alter my operational procedures to make sure that the airplane is not stored with more than a half full tank, but he said that was not good enough. So, it's getting repaired. Since I don't have the repairman certificate, he's the boss.
 
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Once the fuel tank access panel is removed, what is the best way to clean up the outboard rib and the access panel to reseal it?
 
I have a 4 ft length of the 1/4 tubing that the tank vent is made from, it takes a good bit of force to blow through it. I think the holes in the fittings are smaller than the tube ID and the length from end to end of the tube is probably a good bit more than 4 ft with two fittings in it counting the overboard vent fitting. I don't know what the differential pressure from climbing to altitude or descending from altitude does to the internal tank pressure. I don't know if the overboard fitting gets ram air pressure to pressurize the tank either. I also don't know what effect vapor expansion from temperature changes has on the pressure inside the tank. I wish I had made the vents from 5/16 line with thinner wall thickness now. I have fixed tank leaks two times and it is not fun. I just think the resistance to air flow in the lines is causing tank pressure. My Theory, but when I hangar my plane, I crack the fuel caps open so that the tanks can't pressurize. So far, no more leaks. I wish there was an easy way to put a pressure guage on the tank so it could be observed during flight as well as on the ground. Would be good info to know.
 
Worst possible cause?

This is turning into quite a saga. Well, I did get the access plate off, and I did find out what is causing the fuel tank to vent when the tanks are less than full-- The B nut on the inboard end of the right vent line was not just loose, it was inop, because the tubing was either not flared properly, or not flared at all. When I unscrewed the B nut to check the vent line, the sleeve fell into my hand.

Now, I WILL be removing the tank because I don't know if there's a way to repair that vent line without doing that... For that matter, I am not even sure the line can be repaired, I might be looking at a new fuel tank.

Bummer!
 
edneff said:
This is turning into quite a saga. Well, I did get the access plate off, and I did find out what is causing the fuel tank to vent when the tanks are less than full-- The B nut on the inboard end of the right vent line was not just loose, it was inop, because the tubing was either not flared properly, or not flared at all. When I unscrewed the B nut to check the vent line, the sleeve fell into my hand.

Now, I WILL be removing the tank because I don't know if there's a way to repair that vent line without doing that... For that matter, I am not even sure the line can be repaired, I might be looking at a new fuel tank.

Bummer!
Click to expand...

You don't have to make a new fuel tank because the vent line is bad, one end is at the filler, the other at the root rib and you have at least some access to both. Sure it will be a difficult to fix, but way easier than making a new tank. Don't panic. Take the tank off, get in there with a mirror and flashllight, do some head scratching, figure it out. It might be easier than you think. You might not want to smoke while you are doing it though...:p
 
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Thanks for the words of encouragement. I have never used a tube flaring tool, and my mechanic has not had a chance to look at it. I'm hoping he tells me that he can get the tool through the access hole and do the job.
 
To flare it in the tank would be next to impossible with my flaring tool, but based on the RV4 tanks I have just finished building, it would be possible to slide the vent tube out and flare it, then reinstall.

That tubing is very inexpensive. You can watch an eaa video and go make a few practise flares and you will see how easy it is. This ain't rocket science. Just inspect them carefully for cracks and burrs etc.
 
hazard from blockages

If you pressurize the vent from outside and find it blocked momentarily, then it clears up with a pop after some pressure applied by compressed air....
How likely is it that something dangerous is now floating in your fuel? Or sunk to the bottom? Is a camera and bright light inspection in order at this point.
Or just continue to sump fuel from the low point?
 
sblack said:
To flare it in the tank would be next to impossible with my flaring tool, but based on the RV4 tanks I have just finished building, it would be possible to slide the vent tube out and flare it, then reinstall.
Click to expand...

Thanks for the tips. I can slide the tubing inboard towards the access panel opening about 2.5"... more than that and the outboard end will slide inside the outboard rib... I don't think it would be possible to get it back through the bushing/hole in that rib at that point. I think I can get the tubing back inside the clip at the filler tank...

I do have all of the drawings and manual and have been studying them carefully.

I will need a flare tool that will fit inside the access panel opening I think.
 
flightlogic said:
If you pressurize the vent from outside and find it blocked momentarily, then it clears up with a pop after some pressure applied by compressed air....
How likely is it that something dangerous is now floating in your fuel? Or sunk to the bottom? Is a camera and bright light inspection in order at this point.
Or just continue to sump fuel from the low point?
Click to expand...

I don't think there is any obstruction in the tube.
 
I only mention it because I took out the compressor yesterday, after reading the entire thread here... and forced some air through my vents.
Hear a little " phip" (that is a highly technical term) from the left side... and the air traveled more freely. Now I am wondering what might be floating around down there. Might take the little dental cam out and have a look.
Don't suppose those little built in LED lights could cause an explosion???
 
just to close out my contribution to this thread.

My vent line was badly flaired at the inboard connection. My local mechanic removed and replaced the vent line... it was a difficult job. He first threaded a strong flexible wire from the inboard rib through the vent line all the way to and through the fuel filler opening. He then removed the vent line from the inboard side, with the wire anchored at the outboard end. He created a mandrel with a hole through it lengthwise, this was the needle that he put on the leading edge of the new vent line to thread it through the ribs using the wire in place. Vent line installed and plane is back flying. Although I have now discovered some leaks in the right fuel tank... so I will get more practice removing and replacing fuel tanks soon....
 
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