Fuel Trim

Fuel trim is a term used to describe the percentage of correction to basic injection duration based on oxygen feedback. There are two different fuel trim values which affect final injection duration; long fuel trim (Long FT) and short fuel trim (Short FT).

Long FT is after the basic injection duration calculation. It is determined by how closely the fuel system achieves the theoretical air/fuel ratio.

Long FT is a learned value which gradually changes in response to factors beyond the control of system design. For example, fuel oxygen content, engine wear, air leaks, variations in fuel pressure, and so forth.

Short FT is an addition to (or subtraction from) basic injection duration and fuel trim calculation. Oxygen sensor information tells the ECM how close it comes to the ideal air/fuel ratio and the Short FT corrects for any deviation from this value.

   How Short FT Works

Short FT is a temporary correction to fuel delivery which changes with every cycle of the oxygen sensor input. Under normal conditions, it fluctuates rapidly around it ideal value of 0% correction and is only functional during closed loop operation.

Short FT is a parameter on the OBD-II data stream, that can be displayed in the Diagnostic Tester. Its normal range is +/- 20%, but under normal operating conditions rarely goes beyond +/- 10%.

Short FT responds to changes in oxygen sensor input. If basic injection duration + Long FT results in a lean air/fuel ratio, then Short FT responds with positive corrections (+1% to +20%) to add fuel or enrich the mixture. If basic injection + Long FT is too rich, then Short FT responds with negative corrections (-1% to -20%) to subtract fuel or lean the mixture.

When Short FT is varying close to +/- 0%, this indicates a neutral condition where the basic injection duration calculation is very close to stoichiometry, without any significant correction from the oxygen sensor input.

   How Long Fuel Trim Works

Long FT is a data parameter on the OBD and OBD-II data streams. It is a more permanent correction to fuel delivery because it is the correction right after basic injection duration calculation. Long FT changes slowly, in response to Short FT. Its normal range is +/- 20%, positive values indicating rich correction and negative values indicating lean correction.

If Short FT deviates significantly from the +/- 10% for too long, the Long FT shifts, changing the basic injection duration. This shift in basic injection duration should bring Short FT back to the +/- 10% range.

Unlike Short FT which affects injection duration calculation in closed loop only, the Long FT correction factor affects the basic injection duration calculation in both open and closed loop. Because Long FT is stored in NVRAM and is not erased when the ignition is switched off, the fuel system is able to correct for variances in engine and fuel conditions even during warm-up and wide open throttle conditions.

   LEARNED VALUE (Vf) / FUEL TRIM

At the same time the ECM is controlling the fuel injection duration using input of feedback from the oxygen sensor, it is also "learning" about the amount of fuel correction that is taking place. This learned value (Vf) is used by the ECM during both open and closed loop operation to help fine tune the fuel control in relation to engine wear, sensor wear, and air leaks.

   Basic Injection Duration & O₂ Sensor Input

The basic injection duration value is the ECM's best guess at the actual injection time necessary to achieve an ideal fuel/air ratio. Generally this basic injection calculation is very accurate, typically within +/-20% of what is needed. Once within this range, the ECM can trim the air/fuel ratio to stoichiometry based on oxygen sensor input. Depending on many different factors, the amount of correction required for Ox feedback will vary. If the amount of necessary correction remains relatively small, for example less than 10%, the ECM can easily adjust the mixture. As the Ox feedback correction reaches the +/- 20% limit, the ECM fuel correction range becomes limited. The ECM then uses its "learned memory" to adjust or trim the basic injection calculation. By increasing or decreasing the basic injection duration, Ox correction can be held within an acceptable range, maintaining the ECM's ability to correct over a wide air/fuel ratio range.