Fuel Trims — What Are They And How Do They Affect Your Car?

Your car's check engine light can illuminate for many reasons, but fuel trims are one potentially common cause. Fuel trim is a critical part of any modern car's engine management system, but it can also provide a warning that something is amiss with your car. Check engine lights triggered by "lean" or "rich" codes typically originate with excessive fuel trim, but what does this mean, and why should you care?

The Stoichiometric Ratio — Achieving Balance in All Things

Your car's engine is a bit like a big chemistry set. Your engine mixes fuel with air in a high-temperature, high-compression environment to provide your car with the energy it needs to travel down the road. Achieving maximum efficiency and power requires the engine to mix fuel and air in perfectly balanced amounts, known as the stoichiometric ratio.

If there's too much air in the mixture, you would say that your car is running lean. On the other hand, too much fuel means your car is running rich. Even brand-new cars can't maintain a perfect balance at all times, and fuel and air amounts can change depending on engine speed. Modern cars compensate for these imbalances by using fuel trims.

Fuel trims work by monitoring the oxygen content of your car's exhaust gases and using that data to infer the current fuel-to-air ratio. The computer "trims" this ratio by continuously adjusting the fuel available and rechecking the data from its sensors. Since no car will ever have a perfectly balanced fuel-to-air ratio at all times, the computer will typically run a slightly positive or negative fuel trim.

How Fuel Trims Can Alert You to Problems

While fuel trims are a part of your car's normal operation, excessively lean or rich conditions usually indicate a problem. Your car's computer can only add or remove so much fuel, eventually reaching the physical limits of the fuel system. Once the computer goes too far in either direction, it will stop and store a diagnostic trouble code (DTC).

A stored DTC will trigger your car's check engine light, and the underlying code can provide a clue to the problem. A trained mechanic can pull the code to determine if your car is running too lean or too rich and observe the fuel trims to help locate the issue. However, these codes typically do not point to a specific problem. Instead, they provide information about a symptom.

Diagnosing the underlying cause of excessive fuel trims can often be time and labor-intensive. For example, a lean condition may be due to a vacuum leak, problems with the fuel system, or numerous other issues. On the other hand, everything from a leaky injector to a faulty mass air flow sensor can trigger a rich condition.

Because of the complexities involved in diagnosing and repairing these problems, dealing with fuel trim issues is best left to a professional. 

For more info about auto repair, contact a local professional.