ASE Test Prep: Fuel Trims and Filters (VIDEO) -
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ASE Test Prep: Fuel Trims and Filters (VIDEO)

Here are four conditions where the air filter can influence the short-term and long-term fuel trims. Sponsored by FRAM.


Fuel trims are the adjustments the engine computer makes to the fuel mixture to maintain a balanced air/fuel ratio. Fuel trim is usually displayed as a percentage reading or + or – number that will have a fixed range on a scan tool.


A positive fuel trim means that the ECM is adding fuel to keep the combustion range for a stoichiometric range. Conversely, negative fuel trim means the engine is taking away fuel. But, fuel trims have their limits. When fuel trims get too high or low, they will set codes for a bank of cylinders being too rich or lean like P0171 and P0172.

Here are four conditions where the air filter can influence the short-term and long-term fuel trims.

First, a collapsed air filter caused by a clogged wet filter that has become damaged, ripped or dislodged by the vacuum generated by the engine. It will typically flow better than a clean filter. This means it will add fuel to both fuel trims.


Second, if an air filter is made of low-quality materials, pieces of the filter media can be dislodged and block the MAF sensor. They can even damage hotwire MAF sensors. You may get a negative fuel trim number.

Third, if an air filter housing is damaged, cracked or cannot create a seal around the filter, the air may create turbulence or allow unmetered and unfiltered air to get past the MAF sensor. In this scenario, the engine will add fuel for a positive fuel trims.

Fourth, a lot of engineering goes into an air filter. Base fuel trims are calculated using a stock filter. If a replacement filter used the wrong materials, pleat design or gasket material, it can influence fuel trims. In these cases, the fuel trims can be positive if there is less restriction or negative if there is a restriction.


The main concept to remember is that a restrictive filter will cause a negative fuel trim, a missing or damaged air filter will cause a positive fuel trim.

This video is sponsored by FRAM.

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Tomorrows Technician