A PCV (positive crankcase ventilation) valve is fitted to many cars as part of the breather system – especially those systems with a pipe connecting the breather to the inlet manifold.
At low engine speeds this will actually help draw any blow-by gases out of the crankcase and recycle them through the inlet – meaning no oily deposits are dumped on the road.
But if the engine was to backfire the one-way PCV valve closes to prevent the breather system being subjected to the high temperatures of a backfire.
The PCV is also used in turbocharged engines, where a pressurised inlet manifold (under boost conditions) would mean blowing boost directly into the crankcase if a one-way valve were not fitted.
