Quote:
Originally Posted by hackish
On the previous post about the accuracy of these little OBD2 scanners...
The ones that measure injector pulsewidth directly will also need to take into account the battery voltage as the injector opening time is affected by this. This is a tiny difference but over the million or so injections per tank of fuel it can add up to a significant amount. I know this because I did some sub-contracting for a carmaker where they had a fuel economy indicator on the dash - it was approximate at the best of times.
It also takes about 10 seconds to establish an OBD2 connection with an ECU so if you end up with your engine shutoff trick then you may actually be missing the startup and cranking enrichments that the ECU adds while starting the engine. On a cold engine they can be pretty significant - 200% the idle pulsewidth is pretty common. Many cars will also retard the ignition and add extra fuel for the first 15-20 seconds to improve the cat light-off for the cold start emissions some countries mandate.
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Most people doing EOC aren't using the key, they're using a kill switch (I hope it's most, I guess I can't really prove it one way or the other, though). That means they're not killing the ECM, thus not killing the ECM connection.
Information from sensors also doesn't need the ECM to actually be active to be obtained. The Sensors work even when the ECM isn't available, which means that things like the MPGuino (which can take inputs directly from sensors) are getting accurate readings from sensors whether the car is on or not.
Regarding whether or not the methods are "tricking" the gauge into displaying false numbers, that's obviously untrue as well. If all we were doing was "tricking" the gauge into showing a higher number than what we're actually able to achieve, it would show up when we (as many of us do) do the math at each fill up. That's why we 'calibrate' the units upon installation, in some cases.
There's also been an argument posted somewhere that all we're doing is adjusting the instant MPG number. This makes me laugh. If the instant MPG number goes up, it counts into the
average MPG number. The other gauge we're looking at, ya know?
Tell ya what - Raise your Instant Speed a few MPH and tell the cop that it doesn't count because your
average MPH still is lower than the speed limit.