Quote:
Originally Posted by rmay635703
On the buick My FE was stuck around 27mpg on the highway instantaneous (terrible) I noticed my timeing wouldn't move past 25, threw in neutral, shut off motor, turned back on FE jumped after a few to around 37mpg instantaneous and timing was around 37 to think I wasted 15 miles driving at low fe when all it took was a restart.
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Forgive the engineer in me, but it's not 100% clear what actually was going on under the hood. The first question would be 'where is the FE reading coming from'?
If it is from OBD-II, it could be a chicken egg problem. From MAP/RPM/IAT or MAF you can calculate fuel used, but only if you know actual AFR. Modern cars run at stoich a lot, but not always. It is quite possible that a particular MPG gauge uses other clues, like sparkadv, to try to guess when other cases occur.
This raises the question, was the timing really indicitive of poor fuel usage, or suitable for the conditions at hand?
If you are getting MPG from something like injector pulse widths, then we can assume that the injectors never were going into overrun or coast and fuel was being used 'less efficiently' than your normal driving.
But this raises an alternate question, was it really a matter of 'resetting' the ECU, or was the ECU responding correctly to conditions that were altered by shutdown? This is where I find logging a wideband combined with OBD-II data pretty useful. Instead of 'it went away', I can look at the log and get clues for exhaust pressure problems, a clogged injector, bad shift points, etc.
I actually think that even a crappy wideband is pretty useful, because, like EGT, you are looking at the combustion results, what the ECU accomplished. But, unlike EGT, you aren't as easily fooled by abnormal combustion (though a slow wideband can still be fooled, because you are looking at a running average of measurements fed into a PID loop). A fast wideband is nice because you can see, say, individual misfires as spikes in the log. This stuff is impossible to see on a live gauge but when you look at a log you can spot things like a lean cyl or occassional spikes.
None of this says you are wrong, I'm just saying that without knowing how the FE reading was originally derived, and without any engine data to look back at, it is hard to know, for certain, what was really occurring.
-jjf