I still have no perfect solution to the peak and hold injectors without cutting the injector wire. The opamp circuit works just fine and shows ok looking on the scope. It doesn't give a single pulse but a double pulse. When the voltage spikes from the inductive kick it goes high again. So the only thing I have been able to think of is maybe when coasting down with the injectors on just at 1% duty cycle or so the arduino is missing the interrupt signal and not stopping the injector open timer. I set the delay to 0 to try and take that out of the list of things that could cause problems. Didn't seem to do anything.
If missing interrupts is the case then the only choice is to cut the injector wire and put a current sensing circuit in there to trigger the arduino. The signals are pretty short on the scope. A current sensing system could trigger anytime there is current flowing so it would get rid of the problem of the inductive spike making a double pulse out of the signal.
If it is something else I am out of ideas to try
I am probably just going to try building a current sensing circuit and see what that can do for me. It won't be ideal having to cut the injector wire but it at least should reliably read the signals.
For the simple circuits the pot across the zener seems to give good readings as long as the voltage is stable. The opamp gives good readings for various voltages but then goes nuts on very low duty cycle signals. If I can't do any better I think the pot across the zener gives better numbers since the voltage jumps don't seem to be that big of a deal compared to it showing 12-18gph while coasting or slowing down.