What causes a negative fuel trim?

During idle when the throttle plate is closed, the vacuum in the intake manifold is high and very little air flow is entering the engine, so even a small amount of un-metered air will have an effect on fuel trim. This causes the fuel trim numbers to become negative during a boost situation.

Why is my 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.

What should fuel trims be at idle?

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.

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.

How do you diagnose fuel trim?

To test a rich response, you can feed some propane vapor from a small propane tank into the throttle body or a vacuum hose connection on the intake manifold. This time, you should see a drop in fuel trim readings, with STFT going NEGATIVE, and LTFT creeping downward in response to the rich fuel mixture.

Can a lean condition cause a misfire?

“Lean misfire” can occur when the air/fuel mixture is too lean (not enough gasoline in the mixture) to burn. This can be caused by a dirty, clogged or inoperative fuel injector; air leaks; or low fuel pressure because of a weak pump, restricted filter or leaky pressure regulator.

What causes long fuel trim?

Long-term fuel trim relies on sensors further downstream in the exhaust system, past the catalytic converter. While the sensors’ signal will fluctuate frequently upstream, where short-term fuel trim is measured, the signal at the sensors further down should be fairly constant.

What does negative short-term fuel trim 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.