Hi Everyone I wanted to share my experience with leaking fuel caps on my 415 C/D, hope it might help someone else. I've had two separate but similar fuel venting problems and will describe them separately. I have aluminum tanks joined by a balance tube, vented caps, and the return line empties into the right wing tank.
Problem 1) I noticed several times that after filling my wing tanks that fuel started pouring out of the right tank cap vent hole the second I turned the fuel pump on. This was caused from filling the wing tanks too full. There was no room for the fuel being returned to the right tank so as soon as the pump was switched on the right tank would simply overflow. I need to leave the fuel level at least an inch from the bottom of the filler necks. It's easy if I fill the tanks myself but you have to pay attention to line personnel who like to fill em to the top. If this should occur, you don't have to drain fuel, simply delay turning your pump on until after you've flown 10 minutes or so. Just don't forget to turn the pump on! Problem2) I noticed fuel venting into the air while in flight from my left wing tank cap. Also, the fuel was not transferring from the left wing as it should resulting in almost a full left tank, and an empty right tank. I had already replaced the gaskets with little change and the caps appeared to be nice and tight. What I discovered was that even though the caps were tight and the gaskets were new, the tabs on the bottom of the cap were not adjusted to apply EVEN pressure on the gasket. This leads you to believe the cap is tight, when it is actually only tight on one side of the gasket leaving an unsealed area on the opposite side. Take a close look at you gasket when your cap is seated. You can actually use a credit card to slide around the the gasket where it seats on the tank. If you find it loose on one side, this is your problem. Adjust the tabs on the caps by bending them slightly to apply even pressure on the gasket and everything returns to normal. The reason the fuel doesn't transfer when this happens is that with a gasket leak, the low pressure passing above the cap creates a negative pressure in the tank offsetting the positive pressure that the forward facing vent hole is supposed to create. If the left wing is leaking and has negative pressure, and the right wing is sealed and thus has positive pressure as it should, the fuel cannot transfer from left tank(negative pressure) to the right(positive pressure). As a matter of fact, the fuel actually transfers from the right tank to the left. I know that not everyone has vented caps but the same theory would apply. If you create low pressure in one tank by having an unsealed gasket, that tank cannot flow to the opposite side which has higher pressure. I hope this helps. Happy Flying! - Roger - 99399
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