Ford 6.4L Powerstroke Excessive Crankcase Pressure
Some blow-by is normal on any diesel engine. This is because the combustion pressure is just too great for the piston rings to hold completely.
Excessive blow-by can be caused by piston rings that are sticking in the bore. One of the ways to check if blow-by is too great is to put your oil filler cap upside down on the filler hole. If the cap gets blown off, there is too much pressure in the crankcase.
Blow-by can be caused by piston rings that are worn out. A cylinder with worn out rings will have low compression and will likely cause a misfire. These piston rings need to be replaced.