We installed new bladders, fuel pickups, and internal vents recently, but we now have one fuel cap that keeps leaking during flight. We have changed the gasket and believe it is leaking from the vent holes and not around the gasket. Our 185 has two caps per side and only the inner cap seems to be leaking.

We checked the vent lines by attaching a tube on one vent and blocking the other vent. We lightly pressurized the tanks and could hear air going from one tank to the other and then we submerged the line into a bucket of water and the tanks vented, evident by bubbles in the water. Fuel burn is very close after several long flights.

We were thinking we need to raise the vent on the side of the leaking cap to better position it in the vacuum behind the upper strut cuff to lessen the ram pressure. However, we started wondering if we actually need to lower the vent out of the vacuum area. We have checked the fore and aft measurements and they are exactly what the manual calls for.

Any suggestions would be appreciated.



Jeff

Try swapping the caps between left and right sides, see if the problem moves with the cap or stays with that tank.

We tried swapping the inboard and outboard, but on the same wing. We will try swapping the cap to the other wing.

Thanks for the input.

If you want to go all scientific on this it is not too hard. Go find an old junk airspeed indicator. They are essentially a differential pressure gauge, comparing one pressure, static, to another, pitot. If you put a T on the vent lines and run some tubing into the cabin, connect one side to pitot and the other to static. if they where balanced, it would read zero Airspeed. Since negative airspeed is kind of hard to read on those gauges, swap the lines to get a reading if not balanced. Adjust to zero airspeed on the gauge, then it is balanced.

All that being said, for one tank to get enough pressure to vent fuel on the lower filler (which tends to at overflow if the upper is full), then there may be something else. There should be a balance vent line between the two tanks. If not, then it is a very old aircraft or something is amiss on that line. It usually is one of the forward, upper nipples on your bladder, crosses over in the forward part of the cockpit roof.

BTW, old airspeed indicators are good for a bunch of subtle pressure tests, like excessive blow-by on your engine. Use an old cap and put a nipple on it. I had old engine mechs give a specific airspeed for when you need to pull the cylinders for some ring refresh. My limited number of neurons seemed not to have stored the exact number.

Thank you dogpilot. That would be a very good test.

We inspected the vent line during the bladder install and everything was free and clear. When we hooked a line onto the left vent line and pressurized the tank, we could hear bubbles in the right tank. Everything seems to be isolated to one cap (hopefully it is that simple).

Does anyone know of the vents in the stock caps are one-way or bidirectional?

propeller26 wrote:Thank you dogpilot. That would be a very good test.

We inspected the vent line during the bladder install and everything was free and clear. When we hooked a line onto the left vent line and pressurized the tank, we could hear bubbles in the right tank. Everything seems to be isolated to one cap (hopefully it is that simple).

Does anyone know of the vents in the stock caps are one-way or bidirectional?

Pretty sure they are one way. If they were bidirectional the inner caps would always leak when right full of fuel. Theres a good chance that the cap is shot. Does it leak when full and just sitting on the ground?

That is the funny thing, the cap doesn’t seem to leak when the tank is full. I think we will swap the caps on the next long flight to test the theory.

Change the cap. Vented caps are made to let air in, not out; it you've got a reverse flow that's a good sign your cap is Tango Uniform. If the gaskets good you can try blowing the vent out, maybe some crud got stuck on the seal. But a new cap is cheaper than gallons of 100LL.