As I posted a while back, a pollution element failed, had it replaced, and my MPG dropped immediately by around 4 MPG. Since then it has slowly been improving so that after around 5000 miles it is almost back to where it was, with a significant improvement just recently (I took a 1000 mile trip recently...). Tomorrow I take it back in for the 30,000 mile service. So, as someone else posted, the FEH seems to be able to learn to improve MPG. Anyone know how that works?

 

The engine software has both a "short term" and "long term" fuel trim. The fuel is metered into the engine based on the oxygen content of the exhaust gas stream.

After the engine is fully warmed up the fuel is continually varied to give a content from essentially zero to a slight positive amount in the exhaust and back to zero. The effects of the adjustments made to long term fuel trim is what I believe you are observing.

A lot to this subject... calling this a thumbnail sketch overstates the amount of info here.

 

Just to add to Bill's comments, it does not take 5,000 miles for KAM (Kept Alive Memory) to learn Long Term Fuel Trims. KAM is an established memory in your PCM (computer or module) that stores air/fuel ratios for engine loads and RPM's to improve MPG and emissions. KAM can be reset to the default settings by disconnecting your battery cables for about 6 minutes but this does not seemed to be the case in your situation. Your prior thread said your MAF (Mas Air Flow) sensor had to be replaced and this may have caused your problem of a drop in MPG. Your PCM calculates your air/fuel ratios based on your MAF sensor as well as your Heated Oxygen sensors.

With a bad MAF sensor, your air/fuel mixtures go crazy and KAM starts learning and storing all these crazy air/fuel mixtures into maps that now are your new Long Term Fuel memory stored in KAM. Your problem could very well be a screw-up by the tech that changed your MAF sensor. Any time KAM stores bad long term air/fuel maps from a bad sensor, the tech MUST reset KAM to the default settings to prevent long corrections to the crazy air/fuel mixtures stored in KAM from the faulty sensor. This could be one reason it took so long for your long term air/fuel maps to return to where it was before the bad MAF sensor went bad.

There are three other possibilities that will cause a drop in MPG and that is new tires, changing gas qualities (like long trips), and cold weather. Check your tire pressure often also.

GaryG