This modification shouldn't affect engine operation or smog regulations, since the new vent holes are behind the gas cap just like the original vent hole, and therefore closed off when the cap is screwed closed. Also, the filler port now produces a very satisfying 'whoosh' sound as the tank fills at full rate. The results were dramatic! After adding the extra vent holes shown above, the tank could take the highest rate that the station pump will deliver, about 10 gallons per minute. So I tried increasing the size of the existing c-shaped vent hole and adding some extra vent holes as well, using 3/16" and 1/8" drill bits: Even just a little back pressure on the vent would cause the diesel to foam, and back up in the filler tube. On my '97 F250, the filler port vent is the small C-shaped hole in the photo below.Ī few tests determined that this vent hole is far too small for high fill rates. There's a small hole at the filler port that allows the outer hose to vent to the outside. The inner hose delivers fuel from the filler port to the fuel tank, and the outer hose vents air from the tank so that it can fill properly. The filler tube itself is actually one hose inside another. The larger vent at the filler tube is the one that's supposed to vent at a high flow rate during filling. This smaller vent doesn't affect filling at the pump. The check valve prevents spillage in case the truck rolls over. The one on top slowly vents through a check valve as the engine pumps fuel out of the tank. Looking more closely at how the stock tank worked, I learned that there's actually two vents: One on the top of the tank that vents through a small hose to air, and another that vents through the filler tube to the gas cap. I really didn't want to modify or remove the tanks if I could avoid it. There's lots of solutions too, including adding vent lines to the tank or even installing custom aftermarket tanks. Looking for a fix online, I found that many people had run into this problem. A really solid truck! But on the first trip to the gas pump I discovered that both fuel tanks had to be filled very slowly or they would back up and overflow. Recently, I bought a 1997 F250 with the 7.3L Diesel. Simple fix for fuel tank filler vent problem
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