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You can visually inspect used filters once they are removed from the machine just by taking a good look at the filter to see how it appears. For instance, has the filter been damaged? Are the pleats bunched together? Do you see any cuts or damage in the pleats?
Do they look like they are properly sealed on both ends? You might look at the center tube of the filter to see if it has collapsed or buckled. Look for evidence of mechanical damage to the center core of the filter.
You always want to make sure the filter did the job it was intended to do. If it doesn’t look like it has any particles, perhaps it was in a bypass condition. This might be associated with a seal that was pinched, or maybe there isn’t a seal, so you need to verify if the seal was in place and wasn’t damaged.
In addition, when conducting an oil filter inspection, check to see if the filter appears to have been changed on time. If the filter has a matted, dry creek bed appearance, where the particles are packed onto the outer surface of the filter, this might be an indication that the filter was not changed on time.
Oil degradation products like varnish might show up on the filter as well. These degradation products may be seen on the end caps or center tube. Any varnish you find on the filter is an indication of what you are going to find in the rest of the machine.
You also might see sludge hanging off the outside of the filter, which could be black, brown, drab or whitish in color. This is a serious condition that needs to be investigated.
The filter should also be examined for high wear metal concentration. For example, you may have a lot of glittery, metallic particles that are easily visible on the filter. Sometimes a magnet is waved across the filter media to grab onto the ferromagnetic particles. These particles can then be inspected and even put under a magnifying glass.
Other unusual contaminants to look for might include paint chips, fibers, seal material, particles of assorted sizes and colors, etc. Often in machines that have been opened up for an inspection or repair, the exposure to the work environment and the atmosphere can result in particles getting into the system and then showing up on the filter.
Some filters are made of cellulose, which is a wood pulp type of filter media. This type of paper may absorb water. If you have had water in your system, the paper may be swollen or have a wavy appearance.