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Old 09-06-2019, 11:57 PM
Autoputzer Autoputzer is offline
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Originally Posted by RPsX5d View Post
I understand what you are saying . . . but if the car "thinks" it is consuming more fuel (lower indicated mpg) and the O2 sensor is correcting it . . . won't the indicated mpg then creep back towards the actual/manual mpg?

i.e. if the K-factor is off, there will be a constant difference between indicated and manual mpg. What I don't get is the manual mpg stays the same, K-factor has not changed . . . but the indicated mpg is drifting . . .
Because of feedback from the O2 sensors, the fuel injection systems logic is always deciding "I need more fuel" or "I need less fuel." It controls the more or less fuel by adjusting the duration of the fuel injector pulses.

There's a function (equation or graph) that describes how much fuel is delivered given particular duration of the pulse on a perfectly operating injector.

If fuel pressure is low or an injector is restricted, the injectors will deliver less fuel that would be predicted by the function (equation or graph). The feedback from the O2 sensors will correct the lean condition (not enough fuel) by saying "I need me more fuel," and by increasing the duration of the injector pulses. This then provides enough fuel, and the engine runs like it's supposed to (power and drivability, low emissions, and high fuel economy) .... the beauty of feedback!

The MPG displays (on-board and trip) don't really know how much fuel is really being delivered. They only know how long the injector pulses are. They assume the injectors are delivering the amount of fuel as described by the function (equation or graph), and then calculate MPG according to the function (equation or graph).

The K Factor is the ratio of how much fuel was actually delivered (converted to MPG) and how much fuel was theoretically delivered according to the pulse durations and the function (equation or graph) (and converted to MPG). The K factor is used to manually adjust the display, once the technician or car owner knows the ratio of actual MPG/displayed MPG (or displayed MPG/actual MPG).

The fuel injection system never sees the K factor. It just decides "I need more fuel" or "I need less fuel," and adjusts the injector pulse durations accordingly.

If your actual MPG is different than your displayed MPG, that only means that the injectors are not behaving according to that function (equation or graph). But, because of feedback from the O2 sensors, they are still providing the correct amount of fuel.

Last edited by Autoputzer; 09-07-2019 at 12:05 AM.
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