What is normal STFT and Ltft?

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 is normal STFT?

Ideally, the STFT and LTFT should be within a few percentage points of zero when the engine is idling or being held at a steady RPM. Remember, STFT can bounce around quite a bit as when you suddenly snap open the throttle or decelerate. But LTFT can tell you if the average fuel/mixture is running rich or lean.

What should STFT be while driving?

When everything is stable and working correctly, fuel trim numbers should be no greater than 10%, and total fuel trim should be no more than 10% when the numbers are added together. For example, if LTFT is 4% and STFT is 4%, the total is 8%: that’s acceptable.

What causes high STFT?

High STFT are usually caused by a vacuum leak or a bad MAF.

What does negative STFT mean?

A negative fuel trim percentage indicates the PCM is removing fuel while a positive percentage indicates the PCM is adding fuel. A normal STFT reading will generally fluctuate quite frequently, so don’t be surprised to see it as positive one second and negative the next. Usually, they’ll stay within %5 +/- of zero.

Why is my short term fuel trim at?

Short-term fuel trim happens as a result of changes in the oxygen levels of exhaust gases. An oxygen sensor monitors the flow and generates a signal that is proportional to the oxygen levels in the gases. In a properly functioning system, the ECU will react and immediately make changes as the exhaust gases change.

What causes high negative Ltft?

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.

What causes negative fuel trims at idle?

The MAF sensor overestimates airflow at idle, which causes the negative fuel trim values. It then underestimates airflow at cruising speeds, which accounts for the positive fuel trim values.

Is positive fuel trim rich or lean?

Positive fuel trim values If the displayed fuel trim value is a positive number, it means that the ECU is increasing the injector pulse width to add fuel to the air/fuel mixture to enrich the mixture because the input data it is receiving tells it that the mixture is too lean.

What causes rich fuel trim?

Rich. Running rich means that there is too much gas in the air-fuel mixture. This can happen for a variety of reasons, but in a normally functioning system, the ECU will tell it to cut back on the fuel flow to compensate.