Tuesday, July 23, 2024 Detailed Auto Topics
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Most people consider replacing an air filter is good vehicle maintenance. Surprisingly, this is not always so. The air filter we remove may be better than the one we install and installation can cause other problems.

An engine needs air to operate.  They draw air from the atmosphere and mix it with fuel, to run.  Atmospheric air contains a high volume of debris.  Our engine draws in dust and other particles with the air it needs. Engineers design a quality air filter to remove most of the debris, before it enters the engine.  In time the air filter becomes dirty and restricted.  When an air filter restricts the flow of air, we need to replace it.  Replacing an air filter that is still good does not help and careless installation can create problems.

Debris can fall from a dirty air filter into the air intake

No air filter will increase fuel mileage 

To increase filter sales, many vendors state, "Replace your air filter for better fuel mileage." This is totally untrue. No air filter will improve fuel mileage. A restricted air filter may reduce engine power slightly, but they do NOT affect fuel mileage.

All modern engines use computers to precisely regulate the fuel to match the air that enters an engine. The mixture is kept near 14.7 parts air to one part fuel. Restricting air flow causes the computer to reduce the fuel added, not unlike releasing the throttle. Even with an older carbureted engine, reducing air flow decreases the fuel added to the air stream.

We still need to replace a dirty air filter, but it will not increase our fuel mileage. We replace the filter because, dirty air filters reduce engine power and can allow dirt to enter the engine.

Replacing an air filter is simple but should not be taken lightly.  Improperly replacing an air filter can cause problems. Carelessly removing a dirty air filter may allow debris to fall into the air box. Dirt that falls from the filter can then be drawn into the engine.  Most vehicle owners may be careful, but untrained employees at a quick lube may not.

Properly replacing an air filter

When we replace an air filter, first remove all sensors and hoses on the air box. We remove the air box cover and very carefully lift the air filter element out. Be careful to avoid bumping the filter into the air box as dirt may fall out. After we remove the air filter element, wipe the air box, intake hose and all sealing surfaces with a moist towel.

A new, high quality OEM air filter, is placed into the air filter housing and checked for a proper fit. The air filter should push snugly into place, leaving no air gaps are opening. We should not have to push or force the air filter in if it fits properly.

When installing the air-filter-box cover, make sure all fasteners are in place and properly attached. Omitting fasteners may allow dirt to bypass the air filter element and cause wear to the engine. Reattach all sensors and hoses and the job is done. It is also a good idea to attach a small label to the filter housing, with the date and mileage of the last replacement. This helps avoid needlessly opening the air box to check the filter.

Most vehicle owners are careful and properly install their air filters. A big problem is untrained employees at a quick lube or service center. Air filters are an easy add-on sale and they often attempt to sell them. Careless removal knocks dirt into the air intake and improper installation may allow dirt to bypass the filter. An improperly installed air filter harms the engine far more than leaving the dirty filter.

Cheap air filters are very common

The quality of replacement air filters varies greatly. Substandard air filters are very common. A cheap replacement air filter may be far worst than the dirty original filter. Cheap replacement filters often will not properly fit or seal. Substandard filters also allow debris to pass through, due to inferior filter media and seals. Buying an original equipment air filter, from the vehicle manufacturer, is normally the safest practice.


Cheap air filters may not seal and can be worse than the dirty original

The seals on this cheap, substandard air filter have deteriorated. They do not properly seal the element to the filter housing and dirt leaks passed the seals. A check engine alerted the driver after an expensive air flow meter failed from the dirt buildup.

Knowing how much dirt entered the engine or wear the debris caused is not possible. A cheap filter, improperly installed is VERY expensive.

Dirt in an engine and piston rings

oil film protects cylinder walls from wear

To avoid wear, the piston rings ride on a very thin film of oil on the cylinder wall. This thin film keeps the rings from touching the cylinder walls. If we maintain this oil film, almost no wear occurs in the engine.

The pistons move down in the cylinders and draw air through the air filter, and into the engine. With clean fuel and air everything works as designed. Debris that gets through the air filter may bridge the oil gap between the piston rings and the cylinder wall. A particle lodged in this position scratches the cylinder wall and reduces the seal of the piston rings. Keeping the air clean is the job of the air filter.

Engine damage depends on the size of debris that enters the engine

Not all of the dirt that enters the cylinders of the engine will cause damage. Larger particles will not fit in the oil gap, between the ring and the cylinder wall. We expel these from the cylinder on the exhaust stroke. The oil film, on the cylinder walls, captures the very small particles. These are too small to bridge the oil gap and do little damage. Oil carries them to the pan and the oil filter removes them from the engine.

how debris bridges the oil film and causes cylinder wear

Debris that is roughly the same size as the oil film, about four millionths of an inch, causes severe damage. These particles lodge between the cylinder wall and piston ring. This allows them to bridge the oil film. Cylinder wear occurs and the engine will lose power. If enough wear occurs, the rings can no longer control the pressure in the cylinders. The engine loses compression and efficiency falls.

Lubricating oil from the crankcase is also drawn passed worn piston rings, and into the cylinder. The engine burns the oil drawn into the cylinders, reducing efficiency further. This is where the term, "burning oil" originates. When the problem is bad enough, the oil will foul the spark plugs, causing them to misfire. At this stage, rebuilding or replacing the engine is the only repair.

High performance, reusable air filters

They sell many expensive air filters the notion they will increase fuel mileage. This is simply NOT true. Clean or dirty and no matter the design, an air filter will not improve fuel mileage. These filters rely on an oil film to trap debris and must be cleaned on some regular basis. Failure to keep this filter clean, or using too much oil, may damage the air flow meter on the vehicle. The standard paper, original equipment filter is a far better choice to protect the engine.

Needlessly replacing an air filter does no good and can cause all sorts of problems

 Most air filters will last around 30,000 or more miles.

 Only a high quality original equipment air filter should be used.

 Changing an air filter is an important job and cleanliness is imperative.

 Debris from a carelessly replaced filter can damage an engine.

 Changing your own air filter is easy and allows you to control the quality of the job.


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