These vented diesel fuel caps are supposed to limit pressure & vacuum to no more than 2 psi & 2".

Aside from the affects on the fuel injection system, not venting both pressure & vacuum have resulted in either ruptured seams/hoses/fuel leaks or collapsed tanks.

Gasoline systems have been running non-vented caps for quite a while, due to the emissions requirements. In these systems, the tank incorporates a third vent line that is routed to a cannister inside the engine compartment that is designed to capture and recycle gases formed in the fuel system.

Jim