Faulty PCV (Positive Crankcase Ventilation) valve: The PCV valve regulates the flow of blow-by gases back into the intake manifold. When the valve is faulty or clogged, it can cause pressure to build up in the crankcase, leading to smoke coming from the oil filler cap.
Blocked oil return passages: If the oil return passages in the cylinder head or block are blocked, it can prevent oil from draining back into the oil pan, causing pressure to build up in the crankcase and smoke to escape from the oil filler cap.
Worn valve stem seals: Worn valve stem seals can allow oil to leak into the combustion chamber, where it can burn and produce smoke. This smoke can then escape from the oil filler cap when the engine is running.
Head gasket leak: A blown head gasket can allow coolant to leak into the combustion chamber, which can turn into steam and escape from the oil filler cap.
