Fuel Tank Vent System malfunction

L

Larry E. James

New member
While on a trip last weekend I had an issue that required some down-time with a mechanic. In flight I noticed the left tank was nearly full while the right tank was less than 1/2 full. When refueling I noticed an inrush of air when I took off the left fuel cap. Suspecting something wrong with the venting system I found the local A&P and we dug into it. Bottom line is that after a couple hours of undoing/redoing lines and sucking and blowing on various lines; it all checked out good. For the rest of the trip I kept a close eye on the fule levels and all seemed OK. Finally when home and at the final refuel I got the same sucking air thing when I removed the left fuel cap.

I suspect the left tank vent check-valve is sticking. The manual states this is a non-maintenance item. Anyone have any experience with this ?? Does American Champion use more than one supplier for this valve ?? Any ideas ??
 
Larry - I'm curious how you resolved this problem. Our 1973 7GCAA is consistently burning more out of one side than the other. Our mechanic suspects the check valve.

Brian Vulgamore
Scott City, KS
 
Check the condition of the fuel cap gaskets, or just call ACA for a new part and replace them to see if it fixes the problem. Any small leaks in the gasket and it will result in the symptoms you describe.

Paul
 
If one cap is sucking air the opposite tank will drain faster. Try new gaskets or new caps, its nice to have an extra on a cross country trip, trust me(twice).

Terry
 
Larry,

I notice this from time to time. Before you do anything expensive, swap your caps from side to side. Does the sucking sound follow the cap? It does this sometimes on mine.

Bill
 
Yep, I had the same problem a few years ago and a fuel cap fixed the problem; however, I've switched caps and the left tank is consistently burning more... Any ideas?
 
Hello Folks,

I'm new to this message board, but am having the same problem with my 1995 8KCAB. Left tank drains way faster than the right tank. I replaced the cap seals (also tried the swiching of caps, etc), and have had no luck at all. If anyone out there has any thoughts on this, they would be much appreciated.

Thank you
 
Still struggling with differential fuel burn in my Super D (not the fuel cap seals - tried that over and over). I have the vent check valves out. When blowing through them in the "checked" direction, there is a very small amount of air leakage which is actually worst on the faster draining side. Does anyone know if these should be truley air tight in the checked direction? Do you believe this could be the problem?

Thank you,
Sam
 
The check valves need to have a weep hole to allow for expansion. This is the only leaking that should be present.

Since your right tank is drainging slower, is there blockage in the vent line? The right tank has a long line across the leading edge area of the aircraft, unlike the short path that the left tank has.
 
Thanks Jerry, I'll check the right tank vent line, but blowing very lightly throught the pitot easily pressurizes both tanks. That may be way more flow, however, than the normal pitot pressure so could be deceptive.
 
My 1966 7ECA generally has the left tank lower than the right following a flight. My mechanic suggested it might be because the fuel vent is on the left tank facing forward, and this may pressurize the left tank slightly, causing more fuel to flow out of it.
 
I've come to the conclusion that the unveven fuel burn is just "the way it is" with these systems. Unless you have a very dramatic difference or change (ie: all or nearly all fuel is burning from one tank), I believe the cause is a combination of a variety of factors, including; slight variations in pressure loss / drop in vent lines (due to differences in the bends, lengths and cleanliness - tiny but there), differential pressure loss due to condition of tank cap seals (even if they are new), variations in the venting capability of the check valves (they have a very small vent hole so they don't actually seal tight in the "checked" position), difference in time spent yawing right or left during flight (and worst yet, all turns, rolls, spins in one direction - or aircraft out of trim ), differences in fuel line lengths - bends - fittings -etc.

I spoke several times to AC about this, and they assured me that if one tank were to run to "E" during flight, that fuel would be provided from the other tank. That was my biggest concern (taking off thinking you have 40 gallons when you only have 20 available!). I'm not fully comfortable with that, and will never put myself in a situation to "test it", but at least being told by the manufacturer that it won't be a problem does provide a little more confidence.

Bottom line, if you have significant differential burn (significant is a relative word, so you need to use your own assessment - maybe a significant "change" is a better meter), then it should be addressed and could be due to a variety of issues or worst yet, a combination of some the issues noted above (plus things like plugged vents, leaking vent lines or non functional check valves).

If the differences are minimal, we can try to improve via new fuel cap seals, good maintenance of the vent and fuel line systems, and keeping the ball in the middle, but I think we'll have to live with some difference.
 
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