Long Term Fuel Trim and Short Trim Fuel Trim

Long and short-term fuel trim are very useful indicators of engine performance relative to the manufacturer's specifications, but before we get into what they tell you, we'll go over what they are.

Table of Contents

What is Fuel Trim?

Fuel trims are what engine control system use to compensate for all problems relating to air-fuel ratios (known as λ or lambda,) and combustion.

The Electronic Control Module (ECM) or Powertrain Control Module (PCM) uses a few sensors to determine how much air is flowing into the engine. It then uses the airflow data, along with a target air-fuel ratio (λ or lambda) to calculate how much fuel it should inject. The ECM or PCM then uses oxygen sensor readings to find out what the actual air-fuel ratio was, and ‘trims’ the fuel quantities based on this ‘real’ air-fuel ratio.

If the PCM/ECM detects that the calculated quantity of fuel needed is too much or too little (with the oxygen sensor), the PCM/ECM will add or subtract fuel to ‘trim’ the quantity of fuel and optimize the combustion. The difference between the calculated and the actual quantity of fuel being delivered to the cylinders is what is called “fuel trim”. Fuel trim is reported in two different forms: long-term fuel trim (LTFT) and short-term fuel trim (STFT).

fuel trim flowchart

Why do we need Fuel Trim?

The engine controller (ECM or PCM) constantly compensates for changing conditions by adjusting the amount of fuel injected into each port or cylinder. If an engine is operating exactly as designed, the controller can use intake sensors to perfectly predict the amount of fuel that each cylinder will need, for each combustion. Unfortunately, engines are affected by many outside factors, so the controller’s predictions are always wrong in one way or another. ECMs and PCMs use oxygen sensors to detect these errors, and fuel trim to compensate for them. The ‘outside factors’ include:

  • air humidity
  • polluted air
  • fuel issues
  • ignition problems
  • sensor problems
  • engine wear or component breakage

What does Fuel Trim do?

The fuel trim information serves two functions:

  1. To set off the infamous “MIL” when certain limits are exceeded. This will light up to warn the vehicle’s user that there is a malfunction in the engine
  2. To help the technician to identify the cause of an engine malfunction.

How do you Get Fuel Trim Data?

Vehicles manufactured since 2005, which are Global OBD-II compliant and equipped with a CAN system, are required to provide standardized fuel trim data. This is because the OBD standards were created to reduce emissions, and fuel trim is a critical parameter.

To view the LTFT and STFT data from the PCM/ECM you need a scan tool, connected to the OBDII connector on the vehicle (usually on the driver’s side under the instrument panel), while the engine is running. To retrieve STFT information, the engine must be operating in closed-loop mode, which means running it for a few minutes until the oxygen sensor reaches normal operating temperature. It is usually best to monitor the fuel trim under a few different load conditions.

What is the Difference Between Long Term Fuel Trim (LTFT) and Short-Term Fuel Trim STFT?

LTFT is the percentage (%) the PCM/ECM adjusts the calculated quantity of fuel injected into the cylinders to compensate for changes over long periods of time, like clogged fuel injectors, , etc. Changes in LTFT usually take a few seconds to be updated, and are permanently stored in the PCM/ECM memory.

STFT is the percentage (%) the PCM/ECM adjusts (on top of the LTFT) the calculated quantity of fuel injected into the cylinders to compensate adjust to currents conditions. Changes in STFT are usually made a few times per second, and it value is not permanently stored in the PCM/ECM memory.

What are Normal Fuel Trim Values?

Both LTFT and STFT can be positive (more fuel) or negative (less fuel) percentages. LTFT and STFT are added together to produce the total fuel trim, and neither should exceed ± 10% in normal operating conditions.

Fuel trims (LTFT, STFT, or LTFT+STFT) exceeding ±10% may indicate a problem in the engine or the sensors.

Why some engines have two sets of LTFT and STFT?

Fuel trims are adjusted according to the data provided by an oxygen sensor, located in the exhaust manifold. Engines with two banks of cylinders (e.g. H4, H6, V6, V8) have one oxygen sensor per cylinder bank, so each bank’s fuel trims are adjusted according to the data from the oxygen sensor located in its exhaust manifold.

Are “MIL” Warnings Related to Fuel Trim?

If a Long-Term Fuel Trim get too large (±25%), the PCM/ECM will trigger a “MIL” warning, and set one or more Data Troubles Codes (DTC) to help diagnose the problem.  The DTCs directly related to fuel trim problems are:

  • P0170: fuel trim bank 1
  • P0171: system too lean (bank 1)
  • P0172: system too rich (bank 1)
  • P0173: fuel trim bank 2
  • P0174: system too lean (bank 2)
  • P0175: system too rich (bank 2)

What Causes Positive LTFT Values?

Positive fuel trim is caused by ‘leaner’ combustion than the the ECM or PCM predicted. Here are some common causes of positive long-term fuel trim:

  • Air or vacuum leaks in the intake manifold, throttle body or in a vacuum hose.
  • Defective fuel pump not generating enough pressure or volume.
  • Restricted fuel flow due to pinched hose or plugged filter.
  • Defective fuel pressure regulator unable to maintain enough fuel pressure.
  • Air leaks in the PCV system.
  • Malfunctioning MAF (Mass Airflow) sensor.
  • Clogged or unresponsive fuel injectors.
  • Ignition misfires caused by fouled spark plug, weak ignition coil or bad spark plug wires.
  • Damaged exhaust valve that allows unburned oxygen into exhaust.
  • Air getting into the exhaust manifold through a crack or gasket leak.
  • Defective oxygen sensor or its wiring.

What Causes Negative LTFT Values?

Negative fuel trim is caused by ‘richer’ combustion than the ECM of PCM predicted. Here are some common causes of negative long-term fuel trim:

  • Leaky or chattering fuel injector
  • Defective fuel pressure regulator unable to maintain fuel pressure or restricted fuel return line.
  • Restricted air intake system or clogged air filter.
  • Restricted exhaust because clogged converter, crushed exhaust pipe or plugged muffler.
  • Defective oxygen sensor or its wiring.

That’s All!