The air drawn in by the engine is mixed with fuel in the right proportion. To keep dust and grit out of the combustion chamber, an air filter sits ahead of the intake manifold, and on diesel and turbodiesel cars it also protects the air mass meter. Most filters are pleated paper; good quality paper is usually yellowish because it is impregnated with a resin that does not absorb water.
A clogged filter lets less air through and the engine runs worse. Follow the vehicle manufacturer's service interval, but if you drive on dusty unmade roads, in fields or on building sites, check the filter more often.
What you will need
- new air filter to suit your car
- screwdriver or socket for the housing screws
- brush, vacuum cleaner or compressor
- clean cloth
- WD-40 Multi-Use for the fixings
Important: Work on a cold engine with the ignition off. Do not spray lubricant inside the filter housing or onto the filter itself, and never blow compressed air into an open intake duct.
Step 1
Clean the outside of the filter housing
Locate the air filter housing: it is usually high up in the engine bay, sometimes under the engine cover, but always reachable. Clean the outside with a brush or compressed air so that dirt from the engine bay does not fall inside during the change.
Step 2
Open the housing and remove the filter
The lid is held by screws or spring clips. If the screws have seized with moisture and winter road salt, treat them with WD-40 Multi-Use and wait a few minutes so they come out without stripping the heads. Clips simply unhook.
Lift the lid with the intake duct attached and carefully lift out the filter. If the seal has stuck to the housing edge, free it gently so the dust on the filter does not fall inside.
Step 3
Clean inside the housing
Remove dust, leaves and grit from the housing with a vacuum cleaner or compressed air. If you use air, cover the opening that leads to the engine with a cloth. This matters most on diesels: the delicate air mass meter, connected to the engine control unit, sits right after the filter and a strong blast can damage it.
Step 4
Fit the new filter
Place the new filter in the housing without crushing or distorting the paper pleats. An arrow on the filter, if present, shows the direction of airflow. The perimeter seal must sit tightly on the housing edge all the way round, otherwise air will bypass the filter.
Step 5
Close the housing
Refit the lid and secure it with the screws or clips. Once closed, give the screws and clips a light spray of WD-40 Multi-Use on the outside: they stay clean, do not rust and open easily next time. Dispose of the old filter with other vehicle waste.
Step 6
Air mass meter: handle with care
On diesel cars the air mass meter may need cleaning over time. It is a very sensitive sensor: do not touch it with fingers or a brush and never treat it with ordinary lubricants. It is cleaned only with special products approved by the manufacturer, with the connector unplugged and allowed to dry fully before refitting. It is best left to a garage.
Tip: For housing screws hidden under ducts,
WD-40 Smart Straw is handy: the precise jet reaches only the fixing without spraying the filter or the wiring.
What next
With a fresh filter the engine breathes freely. Check it at every oil change and replace it early after a season of dusty roads. Keep the fixings under the bonnet protected from moisture and salt.
These uses come from the experience of WD-40 users in many countries. Always follow the instructions and warnings on the product label.
More guides in «How to use».