I went through this recently. The vent tubes from the cam covers to the air intake downstream from the MAF were broken. Specifically the angle adaptors that interface the plastic tubes to the cam cover, rubberish devices, had rotted out underneath the "knee". This let air get in.
Dorman 46027 PCV Valve Elbow for Select Ford/Mercury Models
from Amazon for $10 fixed it.
You can reach the one on the rear cover from the passenger side using your left hand/arm.
Image (Click to make bigger)
Friday, August 27th, 2021 AT 6:36 AM