When to use STFT or LTFT for fuel trim?

When to use STFT or LTFT for fuel trim?

If so, it automatically provides 5% more fuel (or whatever LTFT adjustment is stored). When reading a scan tool, LTFT won’t change as rapidly as STFT at or near idle. However, if you drive with the scan tool connected, the LTFT will change as the PCM/ECM employs the LTFT adjustments for each cell.

What is the LTFT at idle engine warm?

LTFT +%14 at idle engine warm, this number varies + – %2 or 3. LTFT at 1500rpm no load, +%9. LTFT at 2500rpm no load, +%4. O2 sensor voltage appears correct. Upstream cycles .9 to .1 volts about every 1.5 seconds.

How often does LTFT go on under load?

Driving under load, LTFT is at most + %2-3 O2 sensor voltage appears correct. Upstream cycles .9 to .1 volts about every 1.5 seconds. After cat O2 sensor sit around .7 or .8 unless throttle is changed rapidly. Suspect an air leak but try as I may, none to be found.

What causes a negative LTFT on a car?

Negative LTFT’s with the engine warm, low RPM. In other words, if left uncorrected, the engine would be running too rich. Common reasons for this include over-reading MAF’s, excessive fuel rail pressure and leaky fuel injectors.

What is the maximum fuel trim adjustment for LTFT?

For example, if the maximum LTFT adjustment is +25% and the total fuel trim adjustment is +30%, then LTFT will report +25% and STFT will report +5%, for a total fuel trim value of +30%. The LTFT calculation is kept in memory on most vehicles, so the PCM does not need to relearn the fuel trim calculation the next time the vehicle is started.

What does a decrease in STFT or LTFT mean?

A significant decrease in STFT or LTFT from 0% means the ECU is detecting less air than expected and is adjusting fuel delivery with less fuel. The fuel trims will alter during the above conditions and their altering will provide leads on what is malfunctioning. Each bank has its own fuel trim reading.

Why are the LTFT’s negative at low rpm?

Since the LTFT’s are negative at low RPM, you appear to have an issue with air leaking out after being metered by the MAF’s. Because this is a forced-induction beast, it would be readily explained by a post-turbo, post-MAF leak.

What should my LTFT be on my car?

Each car maker is different, but the general rule is that a LTFT of +25% or -25% indicates a serious problem. Let’s take a look at the possible causes of a high LTFT.