PCV Valve: Operation

When the engine is not operating or during an engine backfire, the spring forces the plunger back against the seat. This prevents vapors from flowing through the valve. See Fig 1.

Fig 1: Engine Off or Engine Backfire No Vapor Flow

Courtesy of DAIMLERCHRYSLER CORP.

When the engine is at idle or cruising, high manifold vacuum is present. At these times manifold vacuum is able to completely compress the spring and pull the plunger to the top of the valve. See Fig 2. In this position there is minimal vapor flow through the valve.

Fig 2: High Intake Manifold Vacuum Minimal Vapor Flow

Courtesy of DAIMLERCHRYSLER CORP.

During periods of moderate intake manifold vacuum the plunger is only pulled part way back from the inlet. This results in maximum vapor flow through the valve. See Fig 3.

Fig 3: Moderate Intake Manifold Vacuum Maximum Vapor Flow

Courtesy of DAIMLERCHRYSLER CORP.