What is a "blow-by"?

Answer 1

Well, here's the story. The crankcase, that which contains the crankshaft and connecting rods, is the bottom side of the engine - you can see the bottom of the pistons from under there. Compressed fuel and air mixture burns in the cylinder on the top of the pistons. When the rings become tired and worn they allow some of this compressed and burning mixture to leak past and escape into the crankcase. That is called "blow by".

In old engines that blowby simply vented into the atmosphere via a tube called a crankcase vent tube. The EPA found bad stuff in that blowby gas and mandated that the engines must suck up that stuff and burn it to render it harmful in some other way. In came the PCV which stands for Positive Crankcase Ventilation, named after the then head of the EPA Pierre Positive.

This system makes the engine suck the junk out of the crankcase via the PCV Valve into the intake manifold and back thorough the system. Welll, if that system is sucking the junk out, there must be a source of air to go into the crankcase else you would just implode the engine and the oil pan would collapse. Soooo, they ran a tube from the top of the valve covers to a source of clean air so that dirt would not get sucked into the crankcase. (note: the valve cover covers the valve train but that is also part of the crankcase - air and blow-by move readily between those zones in the engine.) The source of clean air which was chosen is the air cleaner - makes sense, except for the fact that the air doesn't go thorough the air cleaner filter, so they put a little kotex-like pad in there to clean the bugs and stones out of the air.


unless you are from cape breton or newfoundland , then it is something completely different