P code expert?? What's the trim "system"?

[upallnight]

Originally Posted by btrvalik

As a bit of an experiment, I decided to scan / log my wife's CX-9 to get a better understanding of how a car that doesn't have a code reacts - perhaps a bit apples and oranges, but the basics of fuel and emissions controls seems to be fairly generic - oxygen sensors magnitudes seem to differ a bit between manufacture. Most of the reading see fairly similar to my X5 - for example if I plot MAF / RPM, the plots have the same pattern. The one that jumps out is long-term fuel trim. On my X5 the LT fuel trim will only change once every 10+ min. On the CX-9, it's changing every 20 sec or so.

According to a reference I found on the web:

The PCM’s definition of “short-term” is milliseconds, it acts incrementally, adding or subtracting fuel in small increments in an effort to hit the stoichiometric target.

the PCM is watching what’s happening in response to the changes in STFT. If STFT adds, for example, 3 percent fuel to the baseline and this makes the oxygen sensors happy, the PCM concludes that it ought to adjust the baseline. So it changes the “long term fuel trim” (LTFT). “Long term” is between 30 seconds and a minute. This creates a new baseline.

Based on the above, it would seem my LTFT is changing too slowly - up without knowing what is "normal" it's hard to tell.

@Kevin, Have you tried logging your LTFT? I'm using the Torque App and a BT OBDII which seems to work well.

Next time you get a SES light check the freeze frame data. It will tell you what the engine was doing when it threw up the SES light.