Membranes

The membrane is a thin flexible diaphragm inside the PCV (positive crankcase ventilation) valve cap or oil separator — it regulates crankcase pressure by opening and closing with engine vacuum, letting blow-by gases recirculate while separating out oil vapour. It is the part that fails most often in the PCV system.

Rough or unstable idle, a whistling or hissing noise from the engine, oil leaks from the crankshaft seals or dipstick, increased oil consumption and sometimes a check-engine light — these appear when the cracked membrane can no longer hold vacuum and crankcase pressure rises.

The membrane is thin rubber or plastic that flexes constantly and sits in hot, oily conditions — over years the material hardens, cracks or tears, while the valve housing itself stays intact. That is why replacing just the membrane often fixes the fault without replacing the whole cover.