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Capacitive Fuel Sender Connector Leak

srt

Member
I just finished sealing and pressure testing the first fuel tank and it looks like the only leak is through the BNC connector for the capacitive fuel sender. I've seen note in other build logs that fuel can wick through this connector, so I made sure to cover the back side of the connector in the tank well. The leak is small, I pressurized the tank to about 25" h2o and sprayed with soapy water to find it, small slow bubbles. Could this be air entering the wire at the terminal end and traveling through the length of the wire? Is this something that I need to open the tank to correct?

Thanks,
Scott
 
Unfortunately, only way to rectify that little leak is from the inside, be generous & goop it up good!
Wouldn't hurt to put a dab of sealant at the other end of the wire too.
 
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I have the same problem on my tanks. Have not gotten around to fixing it yet. They both leak right through that little hole in the center of the BNC connector.

I read a few threads here that suggested wicking some green loctite into the connector, but so far that has not been successful for me.
 
If the terminals on the plates weren't sealed, I'd think that *maybe* that could be the source. I covered the plate connections as well as the BNC when I did my tanks.

Unfortunately I don't think there's any way around needing to go inside the tanks, and it'll be really fun if you need to get to the plate connections.
 
Quite possible it's still slipping past the sealant and the wire then finding it's way between the BNC terminal and insulator.

Out of curiosity did you abrade and alcohol wipe the wire sheath and insulator before sealing over?

The one thing goop is not that great at sticking to is Teflon with fingerprints all over it.
 
Wherever the wires are exposed, the fuel can get into the space between the individual strands of wire and follow them inside the insulation to the connector. I don't think there is any way to get around opening the tank to fix this one, but the fix would be to strip away 1/2" or so of insulation next to the connector and thoroughly goop up the wire strands there with proseal to avoid that leak path.
 
Thanks for the tips. I opened up the access panel and will try to goop some more sealant over it. I did clean the wire with MEK, this time I'll abrade it a bit.

Airguy, good tip on stripping away some of the insulation rather than trying to get to the connector. Will that be good enough to get in between the wires?
 
Please forgive my ignorance but something has caught my attention. If fuel/air is making it through the inside of the BNC center pin via the wire braid would it be safe to assume some of you are crimping these connections?

I solder these connectors which should provide for an airtight solid joint filled with solder, assuming proper tinning and technique.

For the sake of sharing experience, on two occasions I've witnessed fuel seeping between the center pin and plastic isolating ferrule that encompasses it. I believe this is the "weak point" of the these connectors, and a great source of comradery for all of us poor souls who have to remedy these leaks. In my case twice. With wings installed.
 
Airguy, good tip on stripping away some of the insulation rather than trying to get to the connector. Will that be good enough to get in between the wires?

And there's the $64,000 question...

It's what I did on mine when I installed them, because I had already heard this sad tale. I took a razorknife and carefully removed about 1/2" of insulation from the wire right at the BNC connector, cleaned the wire with MEK on a piece of paper towel, twisted the wire to open up the twisted strands of individual wires in the bundle, and packed it full of proseal over that exposed section and the wetted end of the BNC connector. They never leaked.
 
Another question I suppose needs to be asked: Does a small air leak from a BNC connector when the tank is pressurized necessarily mean fuel will leak from this spot when a connector is plugged into it? Trying to balance the risk of an actual fuel leak vs the risk of cutting an access hole in the back of the tank to get to the crimped connectors, potentially screwing it up even more.
 
Another question I suppose needs to be asked: Does a small air leak from a BNC connector when the tank is pressurized necessarily mean fuel will leak from this spot when a connector is plugged into it? Trying to balance the risk of an actual fuel leak vs the risk of cutting an access hole in the back of the tank to get to the crimped connectors, potentially screwing it up even more.

Fuel will follow a leak path too small to notice with air and soap bubbles. If you know you have a air leak path, then you also know you have a fuel leak path.
 
This may be a silly question, but are you guys using a hermetically-sealed bulkhead BNC like the Pasternack PE9093 and still seeing leaks, or simply trying to seal up a typical fitting?
 
I had this same problem after flying for about a year. Not enough fuel leaked out to drip, but plenty to make a blue stain. Wicking green locktite in with just a bit of suction on the fuel vent worked for me.
 
My turn with this problem. Flying for 9 years 500 hours on the RV-8. Found the BNC center pin leaking at this years annual. How would one put negative pressure on the tank to try the green locktite trick? How do you keep from messing up the connector with the locktite? I was thinking about opening the access plate and resealing the back of the connector. Also, I was going to strip out about 1/4” of insulation about 1” from the connector as described above but instead of getting proseal in the strands I was going to flux and put a drop of solder on the stranded wire and reseal the insulation with proseal. This should seal the inner sheath leak path.

Thoughts or other ideas?
 
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