It has always been my understanding, that a vented gas cap is vented in one direction, “in” to allow air makeup as the fuel pump empty’s the tank. In addition, the cap seals sloshing fuel and vapors from exiting the tank. This theory is supported by some vented caps that give a warning printed on their cap “Fuel Spray Many Cause Injury, Release Slowly”.
No vented gas cap I've ever owned/used worked that way. In fact, I think a well-vented cap
must allow flow in both directions...at least if it is to work properly. But that flow doesn't need to be much. The fuel use of an inline 6 isn't that high. The original cap of the AD was vented. I have one in the attic somewhere, but in Dallas a locking cap is the smart way to go. As for fuel vapors in the cab, my experience has been this has nothing to do with vented/non-vented gas caps although a cap which does not allow pressure to escape to the outside could exacerbate an existing problem. Fumes generally come from one of three (or maybe more) places...a poorly sealed fuel gauge sender unit will do it. Poorly sealed or deteriorated rubber hoses connecting the fill/vent tubes will do it (less often than the sender gasket, though in my experience). And I've seen in a couple of AD tanks a spot at the top (about 3/8 inch to 1/2 inch long) where the pinch weld simply didn't work. It is very hard to see this unless you know just what to look for, but with gasoline it doesn't take much to stink up a cab. As for sucking the tank in and distorting it, I haven't ever seen that happen. My experience has been the engine will stop running first.
