1998 Lincoln Town Car los of power

My lincoln loses power when under load, ive put new spark plugs in and a new fuel filter. I had a diagnostic done and that reveled some air leaks. Behind the rear wheel there are 2 pipes, they come from a black box at the rear of the car and 1 goes to the fuel tank, the other goes nowhere, should that 1 be blanked off or should it be connected to somthing.