Topic: PCM Reset = >MPG

[Jeffy]

Anyone else notice that when you reset your PCM, your mileage goes up?  At least that's what I've noticed with mine.  I don't notice any other seat of the pants difference but I do see +2MPG after I reset my PCM.  Then when it relearns, it drops back down.

[sharpxmen]

should be backwards, when did you last replaced your O2 sensor?

[Jeffy]

should be backwards, when did you last replaced your O2 sensor?

In the past few years.  It's not bad.

I should mention the engine has been emission dyno'ed and passes.  It's also not the first time I've noticed this.

[sharpxmen]

only other thing i can think of is bad ground reference to O2 sensor, the way i understand it upon reset would be richer as a safety precaution and re-learns so the trims are minimal around 14.7AFR in closed loop so that would make it backwards from what you get - never tried it myself so can't tell for sure. It's an interesting find though.

[neale_rs]

Just going to throw this out for consideration.  Since you are close to see level, your Jeep operates in an extreme point, where the air has a lot of O2.  Maybe the reset PCM mixture (rich at a higher more average elevation) matches well with sea level air.  Also, the 14.7AFR is approximate since what really matters is the oxygen to fuel ratio.  So one question would be if the PCM, with it's sensors, can adjust oxygen to fuel  ratio directly or just the more approximate air to fuel ratio.

[neale_rs]

Another idea.  Maybe you reset the PCM when you have more time on your hands (vacation maybe).  This could lead to atypical driving patterns that could give you different MPG.

[sharpxmen]

closed loop it adjusts AFR several times/second to targer 14.7AFR, in time it learns how much is over/under so the amount of each adjustment is smaller. Closed loop is direct adjustment by reading the o2 output, open loop is always the same as far as i know (regardless if you reset the PCM or not).
When you put the ignition on before starting (cranking) the engine the PCM reads the MAP to get a baseline for where you're at (lower/higher elevation) and uses that information to adjust the fuel. It's a good idea to wait a second or so until all the lights go out to allow for this reading to occur.