What Fuel Trim Values Should Be?

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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.

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