Blowby is caused by the movement of combustion gases in the cylinder getting past the piston rings and into the crankcase area.

A healthy engine's piston rings react to the pressure of the burning fuel and to a great extent are forced outward against the cylinder walls during the power stroke as the piston is pushed down- but as the engine wears , ring grooves, pistons and cylinder walls lose their ability to run within their working tolerances, and blowby increases.

Start your engine, let it warm up and open your oil filler, and you will see it as a vapor coming out of your valve cover.

Some of this is normal, lots of it indicates a worn engine, though in a diesel engine , it's not as critical as in a gasoline engine. Why? Burned blowby leads to carbon deposits that foul the spark plugs, but the diesel just sees it as more fuel and burns it.

__________________
Nate Stanley
(Currently Benzless)
1985 F-250 6.9l 170K
2009 SCION XB 36.5K
2003 LS430 78K
2012 Kubota B 2320