A faulty filler cap would put in a code indicating a leak in the EVAP system. We're looking for a flow problem. Something is restricting the flow of fuel vapor to the intake manifold from the purge canister. Always start with a good visual inspection. Start with the line from the purge control solenoid, located on the passenger side of the intake manifold, to the purge canister, located in the left rear 1/4 panel behind the tire. Remove both ends of the line, you should be able to blow through it very easily. Next, if you plug one end, the line should hold vacuum. You are checking the integrity of the line both for the ability to flow, and also not to leak.