Here's a comparison of the injector signal versus the ECU fuel flow signal. I frankensteined both the injector and ECU fuel flow signals into a stereo audio plug and captured them with my SoundCard Scope. The green signal is from the injector and the red signal is from the ECU.
This is about 30 mph steady on level ground:
You can that there are 6 ECU pulses for every injector pulse as expected.
...and this is about 30 mph uphill accelerating hard.
The ECU pulses appear to be scaled down from the injector pulse width but I expect them to be proportional.
Then I started coasting downhill at about 30 mph...
When coasting, the signals start out looking like the engine is at idle.
...after a couple of seconds, the signal looked like this...
The ECU signal goes completely flat while the injector signal shrinks to a very short pulse. The flat ECU signal may present a problem because the external interrupt routines will not execute.
I compared the injector pulse width versus the ECU pulse width across a range of speeds and throttle settings to get an average ratio between the two. Here's the graph of the relationship:
The Raw data is for the full injector pulse width and the -500uS data is for the inj pulse minus the settling time. So, the ECU pulse width seems to be about 1/8th the width of the injector pulse and it's consistent across different speeds and acceleration.
If I assume that the ratio is 0.123 and I know that the injector uS/gal is 200500000 for my car, when I switch to the ECU signal input, the uS/gal should be 148000000. Calculated as 0.123 * 200500000 * 6 inj pulses. That gives me a starting point to calibrate the ECU signal.
Of course, it still remains to figure out how to handle the ECU flatline signal when coasting.