If equipped with a PCV delete, removing the filler cap will change nothing as the tube to the intake manifold was deleted when a PCV delete or a catch-can was installed.
If it is still PCV equipped, removing the oil filler cap, or pulling the dipstick out a bit, should make it run rough at idle because you've introduced an air inlet leak. Meaning the MAF now reads much less air. Blowing into the PCV-to-manifold tube when disconnected (where circled), only tests the check valve (at the arrow) inside the PCV is containing boost in the manifold when boost is present. If this check valve is failing to contain boost...it will still run rough when the filler cap is removed.
There are still two more valves within the PCV system. The large diaphragm inside the large disc can be torn. Its cap can be removed for access to inspect it. But the cap is delicate and can break or chip while attempting to remove it. Then there is the rear recycling check valve. It is either mounted inside the Rev.-R PCV or mounted in the rear vent tube if using the Rev.-P PCV. The recycling check valve opens to the turbo inlet when there is positive crankcase pressure and closes when crankcase vacuum is present.