Bad valve seals
Worn valve guides
Pressurized crankcase (oil pan) due to a clogged PCV valve or breather system
Blow-by from worn piston rings
Bad valve seals:
The valves are located in thecylinder head above the combustion chamber. Oil is pumped at 50 to 80 psi of pressure into the top of the head, lubricating the valve-train; the valveshave seals on them to stop the flow of oil down into the engine
when the valve is open. If the seals fail, oil is allowed to flow down into the combustion chamber and is burned.
Worn valve guides:
The valves are guided by a small cylindrical chamber called a valve guide. These guides wear over time causing eccentricity (or slop); the excess gap allows the flow of oil down the valve stem into the combustion chamber to be burned. What about the valve seal you say? Well, the gap is too great for the seal to stop the oil flow, so down it goes to be burned.
Pressurized crankcase due to clogged PCV or breather system:
The car's engine is a giant pump, consequently it must breathe. The PCV (Positive Crankcase Ventilation) system does just this, allows the engine to exhaust
the excess pressure build-up (which is a natural phenomenon of the internal combustion engine). Carbon build-up is a by-product of an engine and can build up in the PCV system, clogging the breathing passages. This in turn pressurizes the oil pan and pushes oil up into the fuel delivery system, where it is fed into the engine and burned.
Blow-by from worn piston rings:
The pistons in your car's engine have seals around them in the form of rings. These rings do two things:
Seal the combustion chamber so the precious power developed from the firing of the cylinder is not lost.
Provide vital lubrication to the cylinder walls.
When the rings wear out, the pressure from combustion reverses down into the oil pan, pressurizing it and forcing oil into the valve covers, through the breather system, back into the fuel
delivery system, and into the engine to be burned.
You may ask yourself, "What can I do to stop this from happening?" Keep your oil and filter changed every 3,000 miles and keep the air filters changed every 12,000 miles!! This will keep sludge and carbon buildup down to a minimum. Understand that you can't stop mechanical wear, but you can slow it down!