Engine O&M Guides do not list air filter service interval information because that is not something that is predictable like lube oil, fuel or coolant filter service intervals. Air filters will accumulate dust at varying rates due to ambient conditions depending on terrain, location, traffic, and weather. Because of these varying and complex interaction conditions, air filter service you should be doing is NOT inspecting the filter element to SEE if it is plugged. The most reliable and safe method for determining air filter condition and remaining life is to use a restriction gauge such as the Filter Minder(tm) by Engineered Products of Waterloo, Iowa. This product is now the most commonly used indicator in the engine business, LD, MD, and HD diesel (some gasoline).
How does a Filter Minder Air Filter Service Indicator work?
Restriction indicators are able to measure the small pressure differences between the ambient air pressure and the air pressure on the clean side of the air filter. Most engine manufacturers list the upper limit of air filter life as being 25 inches water, about 1 PSI difference in air pressure. Engine ECM controls measure manifold absolute pressure and adjust fuel rates accordingly. It would appear that most MH operations do not achieve sufficient run time and miles to bring on air filter plugging. If you check your restriction indicator, you may see little movement and conclude that it does not work. This is a common but unfortunate outcome even for large on-highway truck fleet operators who commonly put on 125,000 plus miles per year. For them, with their miles, they frequently change the air filter yearly, whether it is needed or not. For them, that works as they generally keep their units for 4 -5 years then trade them off.
For the MH operator a yearly air filter change is much less desirable as the miles involved are few. Air filters will pass dust when first installed. Usually, it will be during the first 25 hours of normal operation. New air filters are usually 99.3% efficient but after the initial 25 hours the efficiency goes up to 99.8%+ and remains for the duration until the 25 inch restriction limit is achieved. Fewer changes will equate to less dust ingested due to filter service. Any ingested dust is cumulative. Eventually, with enough changes the accumulated dust will reach 4 - 5 ounces, enough to cause cylinder bore and ring wear on a midrange size engine like a Cummins C, ISC, ISL, Caterpillar C9, C12 and comparable engines from International and Detroit.
While it is undesirable to get an air filter wet, the effects are usually short lived. Wetting the air filter will cause the indicator to rise, sometimes to the point of achieving the maximum restriction. It is an easy matter to check the restriction and reset it. Do not change the air filter because of that instance. If it rises to a high restriction level quickly again. Service the element at that time and reset the indicator.
What I am recommending you do for a PM interval for the air cleaner system is to determine that restriction indicator is functioning correctly. If it fails to lock up showing maxium achieved restriction the indicator does you no good. Then you may be forced to change the air filter if you have such doubts.
To test a restriction such as the Filter Minder(tm), operate the engine a low idle speed. Cover the air inlet such that you are inducing an artificial restriction simulating plugging conditions on the air filter. As restriction rises, you should see the indicator rise to some level and hold in that position when you remove the restriction. Shut the engine off. Push the yellow rubber button on the bottom of the Filter Minder(tm). It should drop down to the lowest level. If it does not, the sensing line or small brass fitting is plugged. The small brass fitting on this device has a porous bronze filter. Sometimes they become plugged with dust. There is also a very small orifice hole in the bronze filter fitting. It has been known to plug. If the gauge does not reset, remove it and the brass fitting to clean the fittings. Blow them out with compressed air and use of some solvent or even water. When you re-install the gauge, be careful not to see how strong you are. The Filter Miner(tm) now is made with a straight thread and o-ring seal. It can be hand tightened easily. If you have the earlier style, it has tapered pipe thread. It was easily possible to over-tighten that gauge and crack the housing.
What if the gauge does not lock up during the restriction test? In that instance the gauge is faulty. Replace it.
For some there may be the original restriction gauge from Donaldson. It has a red flag indicator with no means of showing advancing restriction. I would recommend replacing that gauge with the one listed above.
That is all there is to PM interval based air filter maintenance. You should periodically check the rubber boots and band clamps. The rubber boots and elbows on the clean side of the air filter must remain pliable for the band clamps to be able to seal. Due to heat and time, the rubber will take what is called a 'compression set' and become hard. Removal of those boots and connectors and re-installing may not result in the same level of air tight seal that you need to prevent dust ingestion. This is especially important if your air cleaner is a Far Ecolite or any replaceable housing air filter. Tightening the clamp may make the clamp tight but the boot hose does not clamp onto the air housing.