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Fuel Trim

By Ralph Kalal

“Fuel trim” is the amount of fuel added or subtracted by the PCM. The PCM starts out with a baseline fuel/air mixture that it believes will achieve a stoichiometric ratio. When the feedback from the HO2 sensors shows a deviation, the PCM adjusts the amount of fuel the injectors deliver, in an effort to hit the stoichiometric goal. It either adds or subtracts fuel from the baseline.

The amount it adds or subtracts is called the “short term fuel trim” (STFT) and is expressed either as a plus or minus. Because the PCM’s definition of “short-term” is milliseconds, it acts incrementally, adding or subtracting fuel in small increments in an effort to hit the stoichiometric target.  

All this time, the PCM is watching what’s happening in response to the changes in STFT. If STFT adds, for example, 3 percent fuel to the baseline and this makes the oxygen sensors happy, the PCM concludes that it ought to adjust the baseline. So it changes the “long term fuel trim” (LTFT). “Long term” is between 30 seconds and a minute. This creates a new baseline.

The baseline is “injector duration,” which is the length of time the fuel injectors are open, supplying fuel to the cylinders. Both STFT and LTFT are adjustments from the baseline that make the fuel delivery leaner or richer, as needed to best achieve the stoichiometric ratio that represents the most efficient combustion.  

As a general rule, STFT can deviate as much as +/- 20%, but normally should be within +/- 10%. If it deviates by 10% or more for very long, LTFT will adjust the baseline to bring the STFT deviation back within the 10% range. This will then be reflected by a change (+/-) in the LTFT.

To illustrate: suppose the vehicle were operating at 0% correction in LTFT and +10% in STFT when an air leak occurred. STFT initially adjusts for this increase in oxygen by increasing fuel +20%. It does that in 0.6 millisecond. After 30 seconds, LTFT increases +10%. This brings the STFT down to +15%, which is still too much. So, after another 30 seconds the PCM moves LTFT +20%, and STFT drops to less than 10%.  

Long term fuel trim enables the PCM to compensate for variations within the engine itself resulting from production variations, engine and component wear, and minor malfunctions. If you install an aftermarket air filter that is less restrictive than original equipment, it will affect intake airflow. Increased air flow will prompt the PCM to add fuel. It will quickly establish a new LTFT baseline that includes that addition. Similarly, as the air filter becomes more restrictive from the accumulation of dirt and debris, the PCM changes LTFT to adjust for the decreased air flow.  

LTFT also enables the PCM to make the best decisions about the fuel/air mix when it is in “open loop” operation, or is not giving priority at that moment to achieving the stoichiometric ratio. For example, when you suddenly floor the accelerator, the PCM is programmed to recognize that wide open throttle requires a richer fuel/air mixture. But how much richer should it be? LTFT is the data upon which the PCM relies to answer that question.  

Adjustments to LTFT by the PCM are normal. However, fuel trim data may point at a problem elsewhere, so fuel trim data can be very useful in diagnosing engine problems. When LTFT is adding to injector duration, it is compensating for a lean condition. That condition could be caused by an air leak, weak fuel pressure, an exhaust leak in the manifold upstream of the HO2S, clogged fuel injectors, wear in the throttle body, or water in the fuel. When LTFT is negative, there might be a leaking fuel injector, excessive EGR flow, high fuel pressure, or a malfunction in the evaporative emissions system. As with other components of the electronic engine management system, the key to using STFT and LTFT data is in understanding how they affect, and are affected by, other parts of the system.

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