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  #51  
Old 01-28-2012, 07:46 PM
Omega232Devils's Avatar
Omega232Devils Omega232Devils is offline
 
Join Date: Jun 2011
Location: Catawba, NC
Posts: 318
Default 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?"
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  #52  
Old 04-14-2014, 11:00 PM
Chuck McCurry Chuck McCurry is offline
 
Join Date: Feb 2011
Location: Silt Colorado
Posts: 11
Default Venting

I am having the same problem no sulution yet.
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  #53  
Old 04-15-2014, 05:22 AM
terrykohler terrykohler is offline
 
Join Date: Jan 2005
Posts: 1,009
Default 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
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  #54  
Old 04-15-2014, 06:58 PM
flyer174 flyer174 is offline
 
Join Date: Nov 2012
Location: Mosheim Tn
Posts: 30
Default

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.....
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  #55  
Old 04-16-2014, 12:09 AM
Bevan Bevan is offline
 
Join Date: Sep 2007
Location: BC
Posts: 1,674
Default 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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Last edited by Bevan : 04-16-2014 at 12:10 AM. Reason: added link
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  #56  
Old 04-17-2014, 11:30 AM
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bret bret is offline
 
Join Date: Feb 2010
Location: Gardnerville Nv.
Posts: 2,828
Default

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.
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  #57  
Old 04-07-2015, 11:03 AM
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edneff edneff is offline
 
Join Date: Apr 2007
Location: Oakland, CA
Posts: 349
Default 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?
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  #58  
Old 04-07-2015, 11:30 AM
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Mike S Mike S is offline
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Location: Dayton Airpark, NV A34
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Default

Quote:
Originally Posted by edneff View Post

I noted the reference to the "b nut" in the thread. I could not find that in the plans.
http://www.vansairforce.com/communit...ad.php?t=37349
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  #59  
Old 04-07-2015, 12:24 PM
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AndyRV7 AndyRV7 is offline
 
Join Date: Sep 2007
Location: Hudson County, NJ
Posts: 1,092
Default

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
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  #60  
Old 04-07-2015, 12:31 PM
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longranger longranger is offline
 
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Location: 45G, Brighton, MI
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Default

Quote:
Originally Posted by edneff View Post
...

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?
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.
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