Common Filter Questions

Arnie Carlson, field service manager for Donaldson Co., Minneapolis, periodically answers driver questions about air cleaners. Here is a sampling:

Q. I recently inspected a used filter and thought I detected a hole in the media. The distributor said I was wrong. Is there a sure way to detect a hole in filter media?

A. Most filters have areas with low dust buildup — light will come through the media when inspected with a light inside the filter. But buildup can be detected as a bright area where the media is folded and not covered with dust.

Another basic sign of a hole is dust on the liner on the clean air side of the filter. If there is no sign of dust on the clean-side liner of the filter, you can be quite sure that there is no leak in the filter.

Q. Why is there a second filter inside the regular filter in my air cleaner. Is it a spare?

A. Safety filter, secondary element, inner filter, spare element — there are many names, but I prefer to call it a “safety” element. Not a spare, it protects the engine if something goes wrong with the primary element. Until then, it just takes up space and adds a little bit of restriction (1 inch to 2 inches of H2O).

Q. Is there any one thing that causes short service life on air filters? If not, what are some of the major causes?

A. Air filter service life is a pretty subjective thing to measure. Most reports of short life seem to be based on driver complaints of low power. Ordinary contaminants such as dust, dirt, water, chaff, snow, ice and ash, as well as others, can lead to short service life if they are present in quantities greater than the system or vehicle designer originally anticipated.

If a measuring device says the filter is not plugged, believe it and leave the filter alone. If the filter is plugged, change it.