What exactly are fuel trims?

Question

What is meant by the term "fuel trim"?

Why are they so important in diagnosing engine operation issues?

How do short-term fuel trims differ from long-term fuel trims?

Answer 1

Motivation

Fuel trims are a neat mechanism employed by auto manufacturers to reduce the sensitivity of fuel management to operational deviations.

Modern-day fuel-injection systems rely on inputs from many sensors that expect several engine sub-systems to be in tip-top condition. With use and age, many of these sub-systems will experience some degree of degradation, sudden or gradual, that result in the fuel management obtaining a false picture of how the engine is operating.

One such key parameter that the fuel management has to maintain during engine operation is the target AFR (air-to-fuel ratio).

The beauty of fuel-injection management systems is that they don't have to rely solely on sensory input to determine the quantity of fuel which needs to be injected. By using via feedback from the lambda sensors, the fuel management can determine the level of deviation in AFR.

The amount of compensation effected during closed-loop operation is commonly referred to as "fuel trim" (because the fuel injector pulsewidth is controlled ("trimmed") by the fuel-injection management).

It is not uncommon for vehicles to accommodate up to ± 20% fuel trim (± 30 % ± 40 % on some vehicles) before giving up the ghost and throwing an error code/CEL.

Definitions

• A fuel trim of 1.00 or 0% means that there is zero compensation taking place via closed-loop feedback.

• A negative fuel trim (e.g. 0.95 or -5%) means that the fuel injector pulsewidth is being shortened by 5% with respect to what the value should be based on sensor inputs to maintain the target AFR.

• A positive fuel trim (e.g. 1.05 or +5%) means that the fuel injector pulsewidth is being lengthened by 5% in order to maintain target AFR.

• Short-term fuel trims (STFT's) refer to the instantaneous fuel trim correction being applied by the engine management in response to rapid changes in throttle and load.

• Long-term fuel trims (LTFT's) refer to corrections that are "memorized" by the engine management. This will factor in operational deviations like an leak in the intake tract or degraded MAF sensor.

Vital Signs

Fuel trims can be thought of as a vital sign of any fuel management system, much like blood pressure and heart rate is for humans. It can be tracked through most, if not all, OBD-II compliant passenger cars.

By connecting the vehicle to a scan tool it is possible to access live fuel trim data.

With the right signals sent from a scan tool, it is also possible to reset fuel trims.

Example

In this question, the Freeze Frame for a P0300 code reported the following fuel trims:

| Short term fuel % trim - Bank 1         |          0% |
| Long term fuel % trim - Bank 1          |      11.72% |
| Short term fuel % trim - Bank 2         |          0% |
| Long term fuel % trim - Bank 2          |       7.03% |

The lack of STFT correction is an indication that the code was set while the engine was running in open-loop mode.

The LTFT's show that Bank 1 requires more correction than Bank 2; both banks are showing that the average pulsewidth needs to be increased more than what the sensors read in order to maintain target AFR.