What should my STFT be at idle?

Short Term Fuel Trim (STFT) is what the engine computer is doing to the fuel mixture right now. Values normally range from negative 10 percent to positive 10 percent, though the readings may jump as much as 25 percent or more in either direction.

What causes positive STFT?

One thing that restricts airflow into the cylinders is a damaged catalytic converter that causes high exhaust back pressure. At idle this can drive fuel trims in opposite directions, producing a positive STFT and negative LTFT.

What does high STFT mean?

A significant increase in STFT or LTFT from 0% means the ECU is detecting more air than expected and is adjusting fuel delivery with more fuel. A significant increase in STFT and LTFT can be caused by: Unmeasured air is entering the engine due to a vacuum leak (higher fuel trim at idle, but less at higher RPMs)

Can fuel trim cause misfire?

A rich biased O2 sensor may cause inappropriately negative fuel trim on one bank, resulting in a lean misfire. There could actually be too much fuel going to Bank 1, but -35 percent fuel trim is still considered “in fuel control,” so the engine should be running well. However, uneven fueling on Bank 1 could explain it.

Is Ltft bad?

What are Normal Fuel Trim Values? Both LTFT and STFT can be positive (more fuel) or negative (less fuel) percentages. Fuel trims (LTFT, STFT, or LTFT+STFT) exceeding ±10% may indicate a problem in the engine or the sensors.

What causes negative STFT?

Negative fuel trim percentages indicate the PCM is attempting to lean out the fuel mixture, to compensate for a perceived rich condition. STFT and LTFT percentages are the adjustments made by the PCM to maintain the 14.7:1 ratio.

What does a negative STFT mean?

The short-term fuel trim (STFT) refers to immediate changes in fuel occurring several times per second. A negative fuel trim percentage indicates the PCM is removing fuel while a positive percentage indicates the PCM is adding fuel.

What does STFT b1s2 mean?

Member. Those are the catalyst monitors see where it says B1 S2 that means bank 1 sensor 2 you want to see a high % there as it shows no unburned fuel air/gas coming out.

Is Ltft OK?

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.

What causes long-term fuel trim negative at idle?

If the displayed fuel trim value is a negative number, it means that the ECU is decreasing the injector pulse width to subtract fuel from the air/fuel mixture to lean out the air/fuel mixture because the input data it is receiving tells it that the mixture is too rich.

Why is my Ltft negative?

Abnormal persistent negative LTFT might mean an outleak of compressed air (for turbos). But just the value of the LTFT, unless it’s totally out of range, does not mean the engine is running rich or lean. The engine still runs at just what AFR the O2 sensors order it to set.