An air filter should be serviced when the restriction indicator shows that the maximum allowable restriction, which has been established by the engine manufacturer, has been reached.

The filter should not be serviced on the basis of visual observation. A filter that looks dirty may actually be operating at peak efficiency. In other words, don’t be fooled by a dirty-looking filter, as this can lead to over-servicing.

The efficiency of an air filter increases as it is used. As soon as the air filter is put into operation, it begins to remove harmful dust particles. As these particles accumulate throughout the filter media, many of the microscopic openings in the media become obstructed. This on-going reduction in the size of the openings helps the filter stop smaller dust particles, thus resulting in a more efficient filter. As the filter continues to load with contamination, the filter will develop a dust cake that further increases filter efficiency. The restriction to air flow will increase with this dust loading until it reaches the engine manufacturer’s specified restriction level. This defines the dust capacity of the filter element.

It’s never a good idea to clean and reuse an air filter. Cleaning an air filter can lead to the same increased service costs and dust contamination issues as over-servicing, while also decreasing the overall efficiency of the filter and potentially damaging it. Cleaning an air filter decreases the dust holding capacity of the filter by 20% to 40% with each cleaning. Attempting to clean a filter with compressed air or a water hose is also a risky business, as the filter media can become damaged. It only takes one small hole or compromised seal to put expensive engines and equipment in jeopardy.

Engine air filters are designed to actually increase their efficiency by using this initial layer of dust as an added filter layer. Initial filter efficiency is usually approximately 98% but increases to more than 99% by the end of the service life of the filter. Therefore, changing an air filter before the useful service life is achieved can result in premature engine wear.

Results show that clogging the air filter has no significant effect on the fuel economy of the newer vehicles (all fuel injected with closed-loop control and one equipped with MDS). The engine control systems were able to maintain the desired AFR regardless of intake restrictions, and therefore fuel consumption was not increased.

The level of restriction required to cause a substantial (10–15%) decrease in fuel economy was so severe that the vehicle was almost undrivable.

Closed-loop control in modern fuel injected vehicle applications is sophisticated enough to keep a clogged air filter from affecting the vehicle fuel economy.