Just going off what ive seen on the 'net....a breather cap on an EFI car is a no-no.....it will fool the computer into thinking there's a vacuum leak
he's right. the car may run fine and all but you may start to notice idle issues with that setup. The PROPER way would be to plug the PVC system. Look at it this way. The intake manifold is connected to the pvc with obviously is connected to the crank case. The crank case shares the same air with the lifter valley and under the valve covers. The valve cover is vented into the inlet elbow of the factory intake. NOW, when you slap a breather onto the filler neck, you just opened that seal system. The PVC no longer has "suction" on the crank case so you just botched that system. You gain NOTHING NOTHING
by doing this. IF you have oil in the intake, its normally pull in through the PVC. You can installed an air/oil seperator for under 20 bucks.
Either way...IF you go with breathers, you need to block off the PVC completely. Its one or the other.