Excessive Crankcase Pressure or Excessive White Smoke When Removing Oil Fill Cap
Blow-by is an issue that can be caused by different things in you 6.6L Duramax.
Cause: Fuel, air, and moisture forced past the rings into the crankcase.
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.