تلوث زيتي من مرشح هواء مفرط التزييت

Oil from a re-oilable performance air filter carried downstream onto the mass airflow sensor's element. The filter is designed to be oiled, and the fault is quantity: a filter re-oiled generously, or refitted before the oil has wicked in, sheds a mist into the intake.

A coated hot-wire element loses heat more slowly, so the sensor under-reports airflow and the module leans the mixture — and it does so progressively as the coating builds. A lean condition that arrived gradually on a vehicle with an aftermarket filter is this, and it is one of the few faults with a clear preventable cause.

Cleaning the element sometimes recovers it, with the manufacturer's approved cleaner and no contact with the wire, but a heavily contaminated sensor is usually replaced. The important half is the filter: a new sensor behind an over-oiled filter is contaminated again within weeks. Check the ducting for oil film while it is apart, since that confirms the source beyond doubt.