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What are fuel trims?

Article by
Dale Toalston
ASE Certified Technician

There are often questions asked on the forums about fuel trims and I've even had a few emails requesting clarification. Some may have noticed the fuel trim PIDS (parameter I.D.'s) on their scan tool datastream and wonder what they're for. So, what are fuel trims and what do they do? Hopefully we can clear up any confusion. A proper understanding of fuel trims can result in a quicker diagnosis and alert you to future problems with your vehicle.

Basically, fuel trims are the percentage of change in fuel over time. For the engine to operate properly, the air:fuel ratio needs to stay within a small window of 14.7:1. It has to remain in this zone under all the various conditions an engine encounters every day: cold start-up, idling in heavy traffic, cruising down the highway, etc. So, the engine computer is trying to maintain this proper air:fuel ratio by fine-tuning the amount of fuel going into the engine. As it adds or takes away fuel, the oxygen sensors monitor how much oxygen is in the exhaust and respond by telling the computer. The oxygen sensors could be likened to the computer's "eyes" that watch the mixture of oxygen in the exhaust. The computer monitors this input from the heated oxygen sensor(s) continuously when in closed-loop. If the o2 sensors inform the engine computer that the exhaust mixture is lean, the computer adds fuel by lengthening injector pulse, or "on-time", to compensate. Conversely, if the o2 sensors inform the computer that the exhaust is rich, the computer shortens injector pulse, adding less fuel to compensate in order to bring the rich condition down.

This change in fuel being added or taken away is called Fuel Trim. Really, the oxygen sensors are what drive the fuel trim readings. Changes in o2 sensor voltages cause a direct change in fuel. The short term fuel trim (STFT) refers to immediate changes in fuel occurring several times per second. The long term fuel trims (LTFT) are driven by the short term fuel trims. LTFT refers to changes in STFT but averaged over a longer period of time. A negative fuel trim percentage indicates a taking away of fuel while a positive percentage indicates an adding of fuel.

Think of it like this: You're driving from the beach at sea level into the mountains. On a short-term basis, you may go up and down several hundred feet at a time as you ascend the mountainous terrain. But, over the long-term, you are actually ascending from low to high altitude several thousand feet, gradually. It's similar with short and long-term fuel trims. STFT are immediate ups and downs in fuel, while LTFT are what is occurring over a longer period.

A normal STFT reading will generally fluctuate between negative and positive single digits 2-3 times per second. Usually they'll stay around positive or negative 5%, but they may occasionally go up towards 8 or 9% depending on the efficiency of the engine, age of the components, and other factors. A normal long term fuel trim reading will appear to stay the same, giving a long term average of fuel added. It, too, should be close to zero, positive or negative single digits under normal circumstances. It will fluctuate much slower, possibly appearing static.

Normal STFT Reading
Fuel Trim - Normal Condition

If you experience ST or LT fuel trims that are in the double digits positive or negative this would indicate a abnormal adding or lessening of fuel. This could be due to leaking fuel injectors, an unmetered air leak or something similar. For example, if the o2 sensors are reading lean due to, say, a vacuum leak, the engine computer will compensate by adding fuel.

Lean STFT Reading
Fuel Trims - Lean Condition

The STFT will start to climb immediately to reflect the computer adding fuel. While the computer is adding fuel it still watches the o2 sensors until the o2 sensors are indicating that the lean condition no longer exists and proper air:fuel ratio is met. The computer will maintain this heightened addition of fuel until the leak is corrected. The scan tool will show STFT readings that are in the positive double digits, indicating that the computer has been adding too much fuel for normal operation. After a while the LTFT will also reflect this relative addition in fuel. Now if the vacuum leak is bad enough, the computer will not be capable of adding enough fuel to achieve proper air:fuel ratio. It will add fuel until the STFT reaches it's max calibration, usually 25%. Then a code is set in the computer indicating that the engine is running lean (P0171 or P0174) and the STFTs have maxed out. The opposite would be true if an engine was running rich due to a fuel leak (P0172, P0175).

Rich STFT Reading
Fuel Trim - Rich Condition

Keep in mind that the computer has no idea if the o2 sensor is reading properly in some cases. For example if an o2 sensor was sticking rich, the computer would assume it was reading correctly and begin taking away fuel to compensate. This is referred to as a "false rich" condition. The computer would be leaning the engine and setting a possible P0172, P0175. The codes would indicate the engine was running rich but it ACTUALLY is running lean. If you use only the false rich codes to diagnose and don't observe all the fuel trim and o2 sensor data, you may make a false diagnosis.

Also, each bank has it's own fuel trim reading. If your engine is a 4 cylinder, then it has only one bank, Bank 1. On a V-style engine you can isolate which bank is running rich or lean by watching that bank's fuel trims. If one bank is running properly, and another isn't you can narrow down a developing problem to one side of the engine or the other.

Using your new knowledge of fuel trims, you can take the guesswork out of interpreting your fuel system's condition and possibly save some hard-earned money.

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