Air leakage into the crankcase isn't going to affect how the engine runs. Crankcase air is metered into the intake airstream via the PCV valve or PCV orifice. The crankcase has no real vacuum, just airflow from the PCV system and the intake of filtered air opposite the PCV valve/orifice.
The timing cover shouldn't leak air in or out. The issue might be unfiltered air getting into the engine, not the air itself.
The timing cover shouldn't leak air in or out. The issue might be unfiltered air getting into the engine, not the air itself.