PCV Capped off...why?

The way that your car is set up now is the way they used to do it back in the day.

They would run a hose from the valve cover somewhere (the filler neck in your case) up to a plastic adapter that had a piece of open cell foam or some other sort of easy breathing media in it. The adapter was mounted into the air cleaner base. We used to call the plastic adapter assembly a "rag filter". It's purpose was to filter out any oil that the engine would be sending through the vent hose before it got all over the air filter.

At some point the EPA decided that they didn't like the crankcase being pressurized by the exhaust stroke/intake stroke overlap so they mandated the PCV valve into the rag filter setup. This stopped the pressurization of the crankcase but it still wasn't scavenging enough blowby gases from the crankcase.

So they then mandated that the PCV valve should go into the intake manifold itself to get maximum scavenging effect. And that's the way it stands today.

Will it hurt anything to have your crankcase vent setup the way it is? It shouldn't but it could get messy as the rings allow more blowby.

heres the deal, the hose from the TB to the filler neck was rerouted off the TB to the inlet of the blower air box because if it was left stock the pressurized air inside the TB would go into the valve cover pressurizing the crank case causing all kind of problems. the PCV valve itself could be left stock with a hose from it to the back of the intake but most stangs with mileage tend to suck up oil from the crankcase into the intake, so the mod is to cap off the pcv, but when you do this the motor needs some sort of vent so its recommended a breather filter be installed in the valve cover, like the kind that replaces the oil fill cap. hope that helps.