Works best if your DVM has a Duty Cycle function
(I have a 1996 VW Golf with throttle-body fuel injection, i.e. only one injector.)
I think I had similar values when I set my DVM to DC volts. In my car the injector line has on-board voltage (i.e. about 13.5 V) most of the time but gets pulled down to ground periodically to trigger the injector's solenoid. These pulses come with a frequency of twice the engine RPM (4 cylinders, each firing every other revolution) and a duration related to demand.
When you measure with a DC voltmeter this "dirty rectangular wave, pulse-width moduluated" signal gets averaged, so it's a decent measure of what proportion of the time (duty cycle) the injector is on. Keep in mind, though, that when the on-board voltage changes (e.g. when you turn the headlights on) this reading will change. You could probably eliminate this problem by measuring the voltage across a diode (connected in series with, say, a 50 kOhm resistor to limit the diode current).
Many newer or more expensive DVMs can measure the frequency (Hz) and duty cycle (%) of a signal. I bought a relatively cheap one of these (can't afford an oscilloscope) and can now directly measure the injector's duty cycle (and therewith the fuel rate).
I can't remember the exact duty cycle figures but it was something like this:
12% at idle a few seconds after a "cold" start (at 20 C or 68 F)
4.7% at idle when engine warm
10+% maintaining constant speed (60 km/h)
45% during full-throttle acceleration.
One thing became clear: the higher the gear during cruising, the shorter the duty cycle.
So you can indirectly measure momentary fuel consumption (in terms of a rate, so many milliliters per hour). To convert this to mpg or l / 100 km you'd need the vehicle speed signal and (with a microprocessor, usually) divide the fuel rate by speed. That's what ScanGauge and the others do.
It would be nice to have an average fuel consumption, too. Since the ScanGauge doesn't work in my car I intend to do this with my own microcontroller, which is somewhat complicated.
I don't know much about electronics, but you may be able to do an analog average fuel rate simply by smoothing the DC voltage measured at the injector with a big capacitor.
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