Positive Crankcase Ventilation: Description and Operation

Crankcase Ventilation Valve Cross Section:

EVAP System:

DESCRIPTION
A crankcase ventilation system is used in order to provide a more complete scavenging of crankcase vapors. The air cleaner supplies fresh air through a filter to the crankcase. The crankcase mixes the fresh air with blow-by gases. This mixture then passes through a crankcase ventilation valve into the intake manifold.

The primary control is through the crankcase ventilation valve (1), which meters the flow at a rate depending on the manifold vacuum. In order to maintain an idle quality, the crankcase ventilation valve restricts the flow when the intake manifold vacuum is high. If abnormal operating conditions arise, the system is designed to allow the excessive amounts of blow-by gases to back flow through the crankcase vent tube into the air cleaner in order to be consumed by normal combustion.

OPERATION
The Control Module then monitors the ability of the system to maintain the vacuum. If the vacuum remains for a specified period of time, then there are no evaporative leaks, and a PASS report is sent to the control module. If there is a leak, the system either will not achieve a vacuum, or a vacuum cannot be maintained. Usually a fault can only be detected after a cold start with a trip of sufficient length and driving conditions to run the needed tests. The enhanced evaporative system diagnostic conducts up to 8 specific sub-tests in order to detect the fault conditions. If the diagnostic fails a sub-test, the Control Module stores a Diagnostic Trouble Code (DTC) in order to indicate the type of fault detected.