Exhaust pressure under valve cover?

2004 Camry, 4-cyl (16-valve), 130K miles

I posted a few days ago about the oil fill cap’s being left loose after an oil change:

I discovered the loose cap because, immediately after the oil change, I heard exhaust sound from engine compartment and had exhaust smell coming into passenger compartment.

Now, in the middle of the night, I suddenly realize that maybe the oil fill cap (in the valve cover) should not have exhaust pressure behind it. Asking you experts is easier than doing internet research. Is it normal to have exhaust pressure there, or do I have a problem?

Check/replace the PCV valve. It may not be releasing the crankcase pressure into the intake manifold. Also,you may have a bad rubber grommet, not holding it tight enough.

Here’s how to replace it