Fuel coming out of fuel vent

B

brianrussell

New member
I have a 1970 7ECA with the 19.5 gal tanks. Over the last few days I have noticed an intermittent fuel smell in the cabin, in addition the airplane used a ridiculous amount of fuel during a local trip. I refuelled the airplane today and went flying. On approach to another local airport I noticed the fuel smell again. After landing I noticed they were traces of fuel down the fuselage and over the tail surfaces.
From what I can see from the fuel traces, the fuel appears to be coming out of the fuel vent.
Has anyone had this problem or can anyone offer some advice ?
Thanks
B
 
Brian-

Where were the fuel stains on the fuselage? Fuel from the vent should be on the lower wing surface aft of the vent rather then on the fuselage unless it's dripping on the forward lift strut and pooling in the lower strut fairing before flight. My guess is that either the tank or tank to fuselage fuel line is leaking and the fuel is pooling in the wing to fuselage fairing before being sucked out and streaming aft.

How high is the "ridiculous amount of fuel" used, if the leakage is noticeable during a short flight, or in fact any flight, you need to immediately ground the plane and locate/correct the problem before flight.

Tom-
 
Tom,
Thanks for the reply.
The fuel marks (not stains) are on the top of the wing & fuselage on the left side directly inline with the vent and also on the top of the elevator and horizontal stab - left side.They marks from the fuel were not there prior to my flight yesterday.
There is little doubt in my mind that it is coming form the vent, the question would be why ?
I have gone through a lot of older posts and all I have been able to find so far is that the tanks were over full. I will admit that I like to top the tanks right up but this has never been a problem before. I have seen lots of chatter about uneven fuel burn etc and ref made to check valves in the venting circuit so I am wondering if it could be a check valve problem.
I flew the airplane on Friday and when I went to top it up yesterday I noticed that the left tank was M/T (dip stick)and the right tank was less than half (the stick is not yet marked accurately) and this was after 2 hours & 40 minutes of flying.
So while the fuel burn was not as bad as I originally posted it is still higher than normal.
I do not have the fuel smell all of the time, it seems to be more evident on approach (after turns etc) and sometimes on take off.
In fairness I have had a fuel smell at different times right after fuelling and the last couple of top ups I have filled it to the brim.
I have recently switched to mogas and I now have my own pump set up and tend to put as much fuel in as I can, probably even more than when I was at the pumps using avgas.
I have even had fuel expansion during the hot weather after fuelling form cool underground tanks, but not to the extent I noticed on the fuselage yesterday.
In response to your comment about grounding the airplane, I agree 100% and now that I have seen the spillage I will not fly the airplane until it's figured out.I have no desire to check my deadstick landing skills in real time. :)
I posted simply to see if anyone else has ever had a similar problem.
Any and all comments will be appreciated.
Brian
 
There should be an inspection cover pretty much directly aft of the fuel vent. Pull this off and you would probably be looking straight up where the vent is plumbed into the left fuel tank. This would be a good starting point to look for leaks or stains.

Greg B
 
Thanks Greg, I am going to start looking at the airplane tomorrow if I get a chance, but I don't really think this is a leak per say. It appears that fuel is being forced out of the vent during flight. That's why I suspect a faulty check valve. I'll let you know what I find.
Brian
 
Brian,

We had a similar problem several years ago. Fuel would leak after a top off, but would not leak if tank was not full.

We found that one of the tank hold down rubber cushion pads had slipped out from under the hold down strap and allowed the strap to rub a hole in the top of the tank just aft of the filler cap. It would leak when the tank was full, but not when the fuel got below the rubbed hole.

The fuel stain would show up mostly on the bottom and aft of the wing.

I hope this is not your problem as the tank had to be removed to be repaired.
 
Brian-

I can't see how vent fuel would stain the upper surface, are you certain the cap gasket is not cracked or broken...........but similar to a vent leak this should be confined to only one (upper) surface. Ron's suggestion is possible, another common problem is a cracked filler neck from allowing the fuel hose to hang on the neck, which is why I always self fuel. Look for cracks in the fabric to filler neck where fuel could siphon from an internal leak.

Tom-
 
Tom & Ron,
Thanks for the advice. I spoke with my engineer today and his thought is that the tanks were probably too full. He said it is a fairly common problem on Cessnas but has not seen it on a Champion. The check valves apparently will only hold 1/2 PSI so the sloshing in the tank would probably be more that that when the tanks are really full. Don't forget the fuel vent on my aircraft is on top of the wing, not under the wing like the later models. (perhaps this is why it was changed) Anyway, I have washed the fuselage and tail surfaces to get rid of the marks form the fuel. Please keep in mind the marks were not "staining" by any means just marks where I could see where it look like fuel had be splattered down the top left side.
As soon as the weather is a bit better here I will fly the airplane in the circuit for an hour or so (can always use the practise anyway) and we'll see what happens.
Thanks again for your comments.
Brian
 
Brian-

Thanks for the info, I can easily see how a full tank could warm, expand, and overflow on the upper surface and drip to the lower surface before streaming back but I can't imagine loosing enough fuel for it to be noticeable when you refuel, please keep us informed.

Tom-
 
Be sure your fuel cap is sealing properly. A loose or cocked fuel cap can set up a siphon that will suck out A LOT of fuel in a short time. I don't know if your caps are vented or not, but assume not since there is the fuel vent sticking up (like mine). If a cap seal is leaking, it could cause your problem.

Dale
 
Thanks Guys,
The weather looks pretty good for this afternoon so I am going to fly the airplane and see what happens. The major reason I think is may simply be that I really topped the tanks too high is the the fuel marks (not stains) were directly in line with the vent on the top of the fuselage. Also I didn't have any fuel smell in the cabin until I started doing the the approach into a short grass field where the turns were fairly steep. I'll check the caps today before t/o but I think they are OK. I know they are a pain in the butt to remove (very tight) but I will check the gaskets and ensure the caps are seating completely.
I'll let you know what I find.
Brian
 
Hello All,
Well I flew the airplane for an hour yesterday and all went well. No fuel smell and no sign of fuel coming out of the vent. I was in the circuit for an hour so there was lots of change in attitude but all went well.
I checked the fuel caps (before takeoff) and they are both sealing properly. I put a thin layer of grease on the seal and installed the caps and the impression in the grease as well as on the filler neck was complete.
The two events where I have had either a fuel smell or a leak from the vent were when the tanks were topped very close to the top of the filler neck so at this point I have to conclude that when the fuel level is that high the check valves are not able to hold back the fuel sloshing around in the tank and or when the airplane is put into relatively steep turns.
I will no longer fill the tanks to the brim and continue to monitor the situation but all seems to be well.
Most likely just a stupid mistake on my part. At the end of the day filling the tanks to the brim wouldn't give me much anyway.
The whole situation has made me a lot more conscious of my fuel and I have made myself a fuel dip stick so that I can see the exact amount of fuel in the tanks before takeoff.
Thanks to all for the guidance and assistance.
Brian
 
Brian-

Glad to hear all's well, properly coordinated steep turns will not cause cross feed or overflow however steep left slips will frequently cause fuel venting after landing due to cross flow thru the vent line...................it's 100% normal.

Tom-
 
Tom,
Very interesting ! This is news to me and while I believe my turns (steep or otherwise) are coordinated I do slip the airplane often. I'm from the old school that taught if you can't make the field once you cut the power you made a mistake so I am usually high enough that I slip the altitude away to land on or near the numbers. In addition several of the strips I frequent are short and have trees on the approach so again I tend to come in sideways.
I intend to keep the fuel level a little lower but I will also keep your comments in mind the next time I throw the airplane sideways !
Best regards
Brian
 
Brian-

I typically make a rather steep slipping 180 from downwind and fuel venting when the tanks are full is common, somewhat less so when the tanks are less then full but still expected, never a concern and since it's only a small amount of fuel lost I always fill to the top because anything less than a full tank is an unknown.

Tom-
 
Tom,
Your point is well taken. Like you I like to know I have "full" fuel. My point was that the last time I fueled the airplane and had the fuel venting problem I probably went a little overboard (pardon the pun) with the fuel and squeezed every last drop in so that there was just no room left for anything. After that I went flying and certainly did a steep left hand slipping turn to get into the short field.(about 25 minutes form home base)
I read on one of the older posts that Citabria owners should only fill to the bottom of the filler neck.
What do you think ?
Brian
 
Brian-

I have a 2001 7GCAA with the under wing vent and squeeze it full, the left tank always drops slightly below the bottom of the filler neck without venting any fuel, the right tank stays at the bottom of the filler neck. I've tried fueling L/R and R/L and it makes no difference the L always drops just below the bottom of the neck, I assume it's filling the vent crossflow line. I often fill and fly 10 miles to a nearby field and after my signature slipping arrival will see some fuel venting but it is of no concern because I know the cause. If I'm fueling at the hangar in warm Wx and not flying immediately I don't stuff it because I've had it expand and vent in warm Wx and drip down the lift strut and stain the strut and lower strut fairing.

Tom-
 
Tom,
Thanks for the info. Now that I know what's going on I won't stuff it if I am staying local. If I plan to go somewhere I figure I can stuff for max range and keep the airplane straight & level for a while until the fuel has burnt off a bit.
The whole thing just caught me off guard, but as they say "fore warned is fore wise"
Are you going to the east coast fly-in this weekend ? I am going to drive because the weather up this way does not look the greatest but maybe we will ave the opportunity to meet face to face.
Cheers
Brian
 
Brian-

I'm going to make a a social call at Witmer's Fri. afternoon but will miss the main event because I'll be hopping passengers (and chunkin' punkins) at the Golden Age Air Museum (35 miles NW of Pottstown) in the 1929 Waco 10 during annual Pumpkin Drop.

Where are you based?

Tom-
 
Tom,
That sounds like a blast, especially the Waco part !
I am Canadian and I am based out of Cornwall, Ontario (across the river from Massena, NY) I am actually driving down because the weather up here is sketchy all weekend plus the car rental etc. when I get there.
I plan to arrive sometime Friday afternoon so maybe our paths will cross, what's your N number ?
Hope to see you there !
Brian
 
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