Air Filter

Every litre of fuel needs thousands of litres of air to burn, and that air must be clean. The engine air filter, usually a pleated paper or synthetic element in a plastic box near the front of the engine, traps dust, sand, insects and leaves. Filters are replaced at the interval in the service schedule, sooner in dusty conditions. A blocked filter restricts airflow; on modern fuel-injected engines this mostly costs performance at full load rather than fuel economy, because the engine measures the air it actually receives. It is a different part from the cabin (pollen) filter.

Some performance and off-road vehicles use washable cotton or foam filters that are cleaned and re-oiled instead of replaced. Too much oil on such a filter can coat the air flow sensor and upset its readings.

Air enters through a snorkel, passes through the filter element and continues past the mass air flow sensor to the throttle body or turbocharger. The pleats give a large surface so the filter can hold a lot of dirt before it restricts flow. The air box is shaped to calm the incoming air and reduce intake noise. A badly seated filter or a loose box clip can let unfiltered air bypass the element entirely.

A torn or badly seated filter lets grit wear the cylinders and can contaminate the mass air flow sensor, which may cause lean codes and poor running. A badly clogged filter can reduce power and, on diesels, increase smoke. Checking the filter is one of the simplest DIY jobs: open the box, lift the element out and hold it against a light. If light barely passes through, or you see heavy dirt and debris, it is time for a new one.