There are clear operational signs that your air filter has become completely plugged. The engine begins to lose power, fuel consumption increases, and black smoke may blow from the exhaust stack. Continued operation with a plugged air filter may very well damage your engine to the point it may have to be replaced.
It is unwise to trust your eyes when looking at an air filter to determine when a change is necessary. An element that looks relatively clean may be totally plugged with ultra-fine particles from exhaust smoke or air pollutants.
On the other hand, a filter that looks dirty may still have many hours of useful life. Remember that until maximum acceptable restriction is reached, accumulation of dirt on the filter actually adds to the efficiency.
We recommend the use of retraction gauges as an integral part of your engine protection and maintenance program. If you are not currently utilizing restriction gauges, you really should reconsider. Typically, a gauge costs less than a new filter. Here are some benefits: