Can High Crankcase Pressure Can Be Caused by Worn Piston Rings?

All engines naturally experience a certain amount of pressure, owing to a number of factors. Cylinder pressures in a typical engine can easily top 150 to 200 psi during the power stroke; the piston rings keep most of this pressure in the cylinder, but their seal against the cylinder wall isn't airtight. Even if the seal were 99.5 percent perfect -- which it isn't -- the crankcase might still pressurize to about 1 psi. This pressure encourages oil leaks through the gaskets and contributes to air pollution by sending a constant stream of untreated oil vapor steaming out of the engine's breather cap.

"Blow-by" refers to the amount of gas that makes it past the piston rings and into the crankcase. Generally speaking, a street engine will lose about 1.5 percent of the air that goes through to blow-by; or about 1 cfm per 50 horsepower. So, a 250-horsepower engine would see about 5 cfm of blow-by through the oil breather, and a 500-horsepower engine will get about 10. The same 1.5-percent rule applies roughly to pressure. If your cylinder pressure tops out at 150 psi, you should see about 1 psi of pressure in the crankcase.

Worn-out piston rings are half of blow-by equation, since they'll only seal as well as the cylinders themselves. It's common knowledge -- at least among anyone who's ever seen an Engine Restore commercial -- that tiny scratches in the cylinder will allow excess pressure to leak past the rings. But excess cylinder wear does something else too. When the cylinder bore gets larger, the rings extend slightly, losing a bit of their tension and ability to seal the cylinder. This extension also causes the ring gaps to grow slightly, which further encourages cylinder leakage.

All new cars come equipped with a positive crankcase ventilation system, which is essentially just a vacuum tube running from the valve cover to the engine's intake. The slight vacuum in the intake tract offsets pressure in the engine, either neutralizing it or creating a slight vacuum. The PCV system uses a valve mounted in the valve cover to keep engine oil from getting sucked through and into the motor; if this valve malfunctions or gets clogged, the PCV system will fail and you're back to a sealed system. In really extreme cases, pressure in the crankcase can actually push the PCV valve out of the valve cover with a pop like a champagne cork.

Because of the movement of the pistons and the regularity of the combustion events, pressure will typically come out of the engine in regular puffs of pressure instead of a smooth breeze. These puffs can tell you something about the engine's condition. Ideally, these puffs of pressure should be barely noticeable when you hold your hand over the oil filler cap, manifesting as a slight tremble in pressure. The more powerful the individual puffs, the more pressure is spiking with each event and the more cylinder leakage you have. If the puffs are powerful enough to move your hand, it's about time for a rebuild.

The puffs in pressure should be very regular, with no puff more powerful than another. If you feel a few light puffs followed by a single very strong one, then you know you've got excessive blow-by in one cylinder. A grizzled, old mechanic with the mental reaction time of a ninja and a horse-whisperer-like, intuitive feel for engines can actually give a pretty good prognosis on engine condition just by feeling for the frequency and power of those individual pressure pulses. But for those of us who aren't Yoda with a wrench: regular puffs good, irregular puffs bad.

A few things can cause excessive blow-by apart from worn cylinders or rings. Powerful spikes in crankcase pressure are a classic sign of a blown head gasket, or a cracked engine block. This is especially true if the gases coming out of the breather hole carry with them a strong stench of raw gasoline. If you smell raw gas, it's time to hang it up and plan for a rebuild. Leaking exhaust valve seals will also contribute to spikes in crankcase pressure, which are particularly noticeable because the valves are just below the oil filler cap. These puffs will smell more like the exhaust coming out of your tailpipe, with perhaps a slight undertone of additional fuel smell.