Vanne PCV défectueuse ou durite fendue

The crankcase ventilation valve failed, or the hose carrying blow-by gas cracked. Both are unmetered air leaks by definition, which is why this appears so often among the causes of lean fuel trim codes — the air enters downstream of the mass airflow sensor, so the engine gets more air than the module measured.

The valve fails in both directions and they do different damage. Stuck shut, crankcase pressure rises and pushes oil past the crankshaft seals and dipstick tube — which is how an engine that was previously dry starts leaking oil from several places at once. Stuck open, it pulls too hard: oil is drawn into the intake, and in extreme cases seals are sucked inwards, so the engine uses oil with no external leak at all.

Hoses go brittle with heat and age and fail out of sight — splitting along their length or snapping at an elbow, often beneath the intake manifold where nothing is visible without dismantling.

A hose that popped off was pushed off by pressure, so a blocked separator or a stuck valve is usually behind it. Replacing the hose without finding that just repeats the failure. In winter, suspect ice: moisture condenses in the breather and freezes on short-journey cars, blocking it completely.