How to interperet STFT and LTFT scanner data?

Geoff writes’s question:

Just trying to get full use of a OBDII scanner I have just bought. Is the Fuel sys1 and Fuel sys2 information helpful when looking at realtime datastream.”


Here is a little technical info: Using the LONGFT1 and LONGFT2 (Dual Bank Engines) PIDs The LONGFT1/2 PIDs are useful for diagnosing fuel trim concerns. A negative PID value indicates that fuel is being reduced to compensate for a rich condition. A positive PID value indicates that fuel is being increased to compensate for a lean condition. It is important to know that there is a separate LONGFT value that is used for each RPM/load point of engine operation. When viewing the LONGFT1/2 PIDs, the values may change a great deal as the engine is operating at different RPM and load points. This is because the fuel system may have learned corrections for fuel delivery concerns that can change as a function of engine RPM and load. The LONGFT1/2 PIDs will display the fuel trim currently being used at that RPM and load point. Observing the changes in LONGFT1/2 can help when diagnosing fuel system concerns. For example: A contaminated MAF sensor will result in matching LONGFT1/2 correction values that are negative at idle (reducing fuel), but positive (adding fuel) at higher RPM and loads. LONGFT1 values that differ greatly from LONGFT2 values will rule out concerns that are common for both banks (for example, fuel pressure concerns, MAF sensor, etc. can be ruled out). Vacuum leaks will result in large rich corrections (positive LONGFT1/2 value) at idle, but little or no correction at higher RPM and loads. A plugged fuel filter will result in no correction at idle, but large rich corrections (positive LONGFT1/2 value) at high RPM and load. Now you know a little about using these fuel trim figures, BUT really, you could read books on the subject. As all figures in an internal combustion engines feedback with each other, you could graph any data against other engine data and learn some interesting thing. If you want to learn more, visit the autonerdz at where they talk about this stuff all the time. They are crazed auto nerdz and are really into this stuff.

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