Blowby question

[Tony V]

What is it exactly and what does it mean? To check I just take the oil cap off and look for smoke? Thanks

blow by is when the compression inside the cyl above the piston leaks and pressurizes the crankcase. this is mostly from pressures leaking by the piston rings. people say to take the cap off of the oil fill and place it upside down and if it blows off then you have excessive blowby. now on my dually my cap will blow off and when checked at the dealership i was told that it had excessive blow by and to check into a new motor... well i went home and did a compression check and according to my compression my motor is dang near new with all cyl being at 390 PSI +/- 1-2%. only real way to tell is by doing a compression check on all cyls

[Goose350]

The cap will fall off from the engine vibrations if you put it upsidedown so put it in the oil fill like normal just don't screw it down. I have also read that if you put you hand over the oil fill and smoke comes out, that its excessive. It also said if you can feel the pressure that is also excessive as well.