How much is too much fuel trim long term?

Fuel Trim:

Fuel trims are what engine control systems use to complete all problems regarding air-fuel ratios (known as λ or lambda,) and combustion.

The Electronic Control Module (ECM) or Powertrain Control Module (PCM) uses some sensors to work out what quantity of air is flowing into the engine. It then uses the airflow data, together with a target air-fuel ratio (λ or lambda) to calculate what quantity fuel it should inject. The ECM or PCM then uses oxygen sensor readings to search out what the particular air-fuel ratio was, and ‘trims’ the fuel quantities supported this ‘real’ air-fuel ratio.

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

How to Read Fuel Trim?

Reading Fuel Trim:

The fuel trim value is read by plugging a scan tool into the OBD II diagnostic connector located under the electrical device (on the driver’s side near the steering column). When the secret is turned on, the scan tool will initialize and begin to speak with the vehicle’s onboard computer. Betting on the tool and also the vehicle, it should be necessary to enter the vehicle year, make, model, and engine VIN code before the scan tool can read the info.

The engine must be started and running to read the fuel trim information. Betting on the scan tool and the way its menu options are founded, you decide on the choice that enables you to read system live data. This can display a protracted list of sensor outputs and other readings called PIDs (Parameter IDs). On this list are going to be two fuel trim values for inline-four and six-cylinder engines, and 4 fuel trim values for V6 and V8 engines (one pair for every cylinder bank).

Short Term Fuel Trim (STFT) is what the engine computer is doing to the fuel mixture without delay.

This value changes rapidly and might bounce around quite a bit looking at engine load, speed, temperature, and other operating conditions).

Values normally range from negative 10 percent to positive 10 percent, though the readings may jump the maximum amount as 25 percent or more in either direction.

Long Term Fuel Trim (LTFT) may be a future average of what the engine computer has been doing to balance the fuel mixture over a predetermined interval of your time.

This value could be a more accurate indicator of how the fuel mixture is being corrected to catch up on changes within the air/fuel ratio that are occurring inside the engine.

1. STFT B1 is brief Term Fuel Trim engine cylinder Bank 1
2. STFT B2 is brief Term Fuel Trim engine cylinder Bank 2
3. LTFT B1 is future Fuel Trim engine cylinder Bank 1
4. LTFT B2 is a long-run Fuel Trim engine cylinder Bank 2

How does one know which cylinder bank is 1 or 2 on a V6 or V8 engine? Bank 1 is the cylinder bank that has cylinder favorite within the engine firing order.

What Fuel Trim Values Should Be?

Fuel Trim Values Should Be:

Ideally, the STFT (Short Term Fuel Trims) and LTFT (Long Term Fuel Trims) should be in some percentage points of zero when the engine is idling or being held at a gentle RPM (Revolutions Per Minute). Remember, STFT can bounce around quite a bit after you suddenly snap open the throttle or decelerate. But LTFT can tell you if the common fuel/mixture is running rich or lean. Good LTFT values should be as near zero as possible, though they will range from 5 to eight percent looking on the condition of the engine. If the LTFT is getting up around 10 percent or higher, it always indicates an issue that must be diagnosed. LTFT values that arise around 20 to 25 percent will usually set a P0171 or P0174 lean code. LTFT values that drop to negative 20 to 25 will usually set a P0172 or P0175 rich code.  

Fuel Trim Values Mean:

POSITIVE fuel trim values mean the engine computer is adding fuel (increasing the heartbeat width or on-time of the fuel injectors) to feature more fuel to the engine. In other words, it’s attempting to RICHEN the fuel mixture because it thinks the engine’s air/fuel mixture is running too lean. NEGATIVE (-) fuel trim values mean the engine computer is subtracting fuel (decreasing the heartbeat width or on-time of the fuel injectors) to scale back the number of fuel injected into the engine. This is often done to LEAN out the fuel mixture to atone for what it perceives as an expensive running condition. Remember, all this is often supported by what the oxygen sensors are telling the engine computer. If the O2 sensors indicate LEAN, the pc adds fuel and generates a POSITIVE fuel trim value. If the O2 sensors are reading RICH, the pc compensates by subtracting fuel and generates a NEGATIVE fuel trim value.

What Causes Positive/Negative LTFT Values?

Causes of Positive LTFT Values:

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

1. Air or vacuum leaks within the manifold, throttle body, or during a vacuum hose.
2. A defective fuel pump not generating enough pressure or volume.
3. Restricted fuel flow because of a pinched hose or plugged filter.
4. Defective fuel pressure regulator unable to take care of enough fuel pressure.
5. Air leaks within the PCV system.
6. Malfunctioning MAF (Mass Airflow) sensor.
7. Clogged or unresponsive fuel injectors.
8. Ignition misfires are caused by the fouled plug, weak induction coil, or bad sparking plug wires.
9. A damaged valve that permits unburned oxygen into the exhaust.
10. Air moving into the manifold through a crack or gasket leak.
11. Defective oxygen sensor or its wiring.

Causes of 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:

1. Leaky or chattering fuel injector
2. Defective fuel pressure regulator unable to take care of fuel pressure or restricted fuel return line.
3. Restricted air intake system or clogged filter.
4. Restricted exhaust thanks to the clogged converter, crushed pipage, or plugged muffler.
5. Defective oxygen sensor or its wiring.

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