See Figure 1
When the engine is running, a small portion of the gases which are formed in the combustion chamber leak by the piston rings and enter the crankcase. Since these gases are under pressure they tend to escape from the crankcase and enter into the atmosphere. If these gases are allowed to remain in the crankcase for any length of time, they would contaminate the engine oil and cause sludge to build up. If the gases are allowed to escape into the atmosphere, they would pollute the air, as they contain unburned hydrocarbons. The crankcase emission control equipment recycles these gases back into the engine combustion chamber, where they are burned.
Crankcase gases are recycled in the following manner. While the engine is running, clean filtered air is drawn into the crankcase through the intake air filter and then through a hose leading to the oil filler cap or the valve cover . As the air passes through the crankcase it picks up the combustion gases and carries them out of the crankcase, up through the PCV valve, (Volvo calls it the flame guard) and into the intake manifold. After they enter the intake manifold they are drawn into the combustion chamber and are burned.
The most critical component of the system is the PCV valve. This vacuum-controlled valve regulates the amount of gases which are recycled into the combustion chamber. At low engine speeds the valve is partially closed, limiting the flow of gases into the intake manifold. As engine speed increases, the valve opens to admit greater quantities of the gases into the intake manifold. If the valve should become blocked or plugged, the gases will be prevented from escaping the crankcase by the normal route. Since these gases are under pressure, they will find their own way out of the crankcase. This alternate route is usually a weak oil seal or gasket in the engine. As the gas escapes by the gasket, it also creates an oil leak. Besides causing oil leaks, a clogged PCV valve also allows these gases to remain in the crankcase for an extended period of time, promoting the formation of sludge in the engine.
Servicing the crankcase ventilation system consists of checking the hoses for cracks or vacuum leaks and checking the hoses, calibrated nipple and flame guard for clogging. Remove the hose from the air inlet tube and with engine running, check for a presence of vacuum. If no vacuum is present, find the restriction in the PCV system.
REMOVAL & INSTALLATION
Refer to General Information & Maintenance for removal and installation of the PCV valve. The PCV nipple should be removed and inspected every 60,000 miles (96,000 km).