At idle, the crankcase pressure is much higher than the intake manifold pressure and the valve opens to let the gases through. At higher loads, the intake manifold pressure reduces.

What causes crankcase pressure high?

If the engine is producing blow-by gases faster than the PCV system can dispose of them, an increasing surplus becomes trapped in the crankcase, causing excess pressure and, inevitably, oil leaks. Even the most carefully sealed gaskets leak when confronted by rising internal crankcase pressure.

What should the crankcase pressure be?

On engines using the factory designed crankcase ventilation system (a PCV or “positive crankcase ventilation” system), we typically measure peak crankcase pressures on the order of 2.5 to 6.0 psi when the engine is in normal running order.