Anyone know anything about short term fuel trims?

This evening I took a 60 mile (30 each way) trip and each way STFT2 had high values 18 to and 15.6 on the return. Misfires happened mostly on cyl#6 almost 4k each way (total of 8k for the duration of the trip.

Any suggestions, things I could check?

 

A positive STFT percentage indicates a lean condition and that the PCM is adding fuel by increasing the injector pulse width on that bank to bring fuel mixture back to 14.7:1 The misfires on cylinder #6 is a pretty good indication that cylinder #6 is running lean. Most likely causes are a vacuum leak or an injector not opening properly on cylinder #6. If it's a vacuum leak, it will be specific to that cylinder only, ie not something that affects all cylinders. In other words, look for a vacuum leak very near the intake runner for cyl #6.

A problem with a spark plug, plug wire, or the injector stuck open, (or not sealing) would all cause a negative STFT percentage and the PCM would decrease injector pulse time.