Positive crankcase ventilation (PCV) behavior The PCV valve uses a spring-loaded plunger. Does it open in direct proportion to intake-manifold vacuum level?

Introduction / Context:
Positive crankcase ventilation meters blow-by gases from the crankcase to the intake to reduce emissions and sludge. The valve must meter flow correctly across idle, cruise, and wide-open throttle (WOT) conditions.

Given Data / Assumptions:

• PCV valve is a spring-biased, vacuum-responsive metering device.
• Intake manifold vacuum is high at idle/overrun and low at WOT.
• Goal: prevent excessive lean condition at idle yet provide adequate scavenging at load.

Concept / Approach:
At high manifold vacuum (idle), the strong suction pulls the plunger against its seat, restricting flow (more closed). At low vacuum (WOT), the plunger opens wider to maintain ventilation. Thus, opening is inversely related to vacuum level, not directly proportional.

Step-by-Step Solution:
Idle/high vacuum → valve nearly closed → small metered flow.Cruise/moderate vacuum → valve opens to a mid position.WOT/low vacuum → valve opens further to vent blow-by while avoiding backflow.

Verification / Alternative check:
Bench test: apply vacuum to a PCV valve and observe pintle movement; more vacuum pulls it toward closed, confirming inverse relation.

Why Other Options Are Wrong:
“Agree” reverses the actual relationship.

Opening is not limited to deceleration or WOT only; it modulates continually.

It is not fully open all the time; modulation is essential to mixture control.

Common Pitfalls:
Clogged PCV can cause oil leaks, sludge, or idle mixture issues; wrong valve calibration can upset fuel trims. Always verify hoses and orifices for restrictions.

Final Answer:
Disagree