Fuel leaking from aux tank?
 

Hello ALL. My '65 C337 has been leaking fuel from the aux tank scupper and from a rubber hose under the wing near the inside(small) flap( possibly aux tank vent line). I studied the parts manual. I filled all the fuel tanks the other day and the left aux tank started leaking from the above mentioned locations. I place a bucket under the wing . Today I went back to check on my bird and the leaks had stopped after filling a three gallon bucket. I checked the aux and left fillers, the aux was FULL to the brim, and the left main was down about 5-6 gallons???? I'm pretty sure there is no direct connection between the aux tank and the main tank, except maybe through the sump tank. I'm wondering if it's something to do with the check valves in the scubber line and or the aux tank vent? But how would fuel from the main entering into the Aux tank? Also there is blue fuel stains under the left wing and down the left side of the fuselage? Any advice would be appreciated!
Regards, BILLS

Fuel valve. Your fuel valve is located at the underside of the wing where it joins the fuselage underneath an access plate. Worth a look to see if you are having some leaking there.

Right now my fuel valve is not providing positive cutoff of fuel flow. A small trickle happens in off position. This tells me the ball in the valve is not sealing all the way. (a project for the near future). Likewise, if the balls in your fuel valve are not providing positive seal for the ports to aux tank and main tank, they end up being in communication. So fuel can trickle or flow from one tank to the other; they are in communication at the valve.

So why would fuel move from main tank to aux tank? Possibly a pressure differential. Specifically the main tank getting pressurized. Possible fuel vent plugging. It's getting warmer now, fuel heating up and fuel vapor pressure going up. Check fuel vent for main tank.

I had a visit today from a fellow curious about my skymaster while I was working on it. He went on to tell a story of about two episodes of engine stumble and power loss in his plane. Long story short he had fuel vent plugging from dirt dauber nest. He cleared it by using weedeater line and measured to be sure weedeater line reached all the way to his tank. When he saw mud on the end, he lightly pressurized his tank blowing out nest fragments.

Not too long ago I cleaned my scupper drains which were plugged by dirt daubers allowing water to get into tank. Shortly after one scupper had water pooled in it again. I probed the drain and found a fresh insect larvae blocking it. Didn't take long for a bug to find it again. They like those vent and drain pipes.

Fuel leaking from aux tank?
 

WSlade2.
Thanks for the reply. I'll check the vents and scrubber lines, also the main tank vent valves. The selector valves may need a look at as well.
Regards, BILLS

I have the same issue if I leave my valve on "main". The dihedral of the wing places the main tanks higher than the aux tanks. That will push the fuel through the aux and out the overflow if your valve does not seat properly or if valve is out of sync with the selector handle. Mine stops if I turn the valve to "off"

Fuel leaking from aux tank?
 

Hello ALL.
Has anyone used the fuel sender unit gasket S-1588-'2' which is thinner than the original S-1588-'1'? McFarlane has them for $23 each VS $36 for the original? Their gaskets are make with Vilon a special material for fuel environments??
Thanks, BILLS
PS: Cessna wants between $80-$104 each,FYI.

The parts manual that I have (1965-1969) calls out the -1 which is what I got from McFarlane when I replaced mine. I try and stick with what the factory calls out as much as possible and makes sense. The price difference isnt all that much as you noted - albeit that you need 6 of them for a complete job.

The McFarlane parts I have used so far are have been great quality and economical. Heck, they even have an "expiration date" on the fuel level sender gaskets to help control quality.

My recommendation would be to stick with what was originally used.

Fuel leaking from aux tank?
 

To ALL. FYI, I have found a gasket manufacturer that will make the S1588-1 fuel sending unit gasket with the popular material VITON, the same as McFarland's. For about $25 each VS McFarland $36. If you replace all six gaskets it's a good savings. Currently this gasket is NOT FAA/PMA but with interest the guy said he'll go through the PMA process. The same company currently offer fuel cap gaskets for many aircraft types, including Skymaster, with FAA/PMA certificates for around $8.
Use at your own risk. Just trying to save you guys/gals some money.
Thanks, BILLS