Ventilación del cárter defectuosa que introduce humedad en la admisión

A crankcase ventilation system passing water vapour and oil mist into the intake instead of separating it. The crankcase always contains water — a product of combustion that condenses on cold metal — and the ventilation system is supposed to draw it off and burn it once the engine is warm.

Short journeys are what defeat it. An engine that never reaches full temperature never evaporates the condensate, so it accumulates as an emulsion, and the breather passes moisture into the intake where it fouls the airflow sensor and can freeze in cold weather. A mayonnaise-like deposit under the oil filler cap is the classic sign, and it points at usage rather than at a head gasket in most cases.

The consequences reach the sensors. A wet airflow element under-reports, an intake temperature sensor responds oddly, and in freezing conditions ice in the breather can pressurise the crankcase and push oil past a seal. Check the breather's separator and any heater fitted to it, and treat the driving pattern as part of the diagnosis.